Just install it and let it install GRUB. Then if you want to make the GRUB of your first install the primary one again, boot into that install, make sure os-prober and the menu are enabled in /etc/default/grub, and run… 
sudo grub-install --recheck && sudo update-grub
More about my system.
Main Manjaro is on sda2
lsblk -l
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 465,8G 0 disk
sda1 8:1 0 300M 0 part /boot/efi
sda2 8:2 0 97,7G 0 part /
And inside sda2 partition GBUB files are located in /boot/grub/
(and I do not have any separate boot partition).
Now I want to install the second, small and “emergency” installation of Manjaro on /dev/sda5
sda5 8:5 0 14,8G 0 part`
Currently on this partition there are no GRUB files (a directory /boot/grub/ is empty)
So when during the installation on sda5 I will set:
sda1 for /boot/efi
sda5 for /
will installer reinstall all GRUB files located in /boot/grub/ on sda2?
Stop talking about grub, this is all about the boot partition.
Follow what Aragorn said in the last post.
Grub will detect BOTH INSTALLATIONS.
Your old installation will never even be mounted, it has nothing to do with the new install.
The grub files you talk about will be installed at the same place but on /dev/sda5
The grub you keep talking about is just the program creating the boot menu.
Edit
Make sure you have following in your /etc/default/grub (on both installations) so the menu is actually visible
GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=menu
Then sudo update-grub
Erm, it does matter. It is about grub.
If its EFI, then its the grub installed to the ESP, but grub nonetheless.
Which is why the guidance is:
Which is reasonable advice because whichever grub you do that to last will be “the” grub.
(Well, I would normally do something like sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck, but I assume aragorn has a reason for the suggestion as is)
OK.
I do not have any separate boot partition and files used by GRUB are on sda2 in /boot/grub/ .
And installing the second system on sda5 will create files used by GRUB are on sda5 in /boot/grub/ .
Now I will do as @Aragorn wrote to use main system on sda2 and will be using this system daily.
But let’s assume in few months I will want to update the second, emergency system on sda5.
As I understand this update of system on sda5 will result that the system on sda5 will became the main system (first seen in GBUB) - what will give more complications.
That’s why I wanted to install a second system without GRUB.
Or can I do an update on sda5 without changing the GBUB files?
Silly question:
Why an Emergency Install of Manjaro, why not backups?
Backup needs more space, which I do not have.
And emergency system is a convenient replacement for live USB in emergency situations.
Yes you do.
You want to update the second more often than after 5 months. On stable, I would wait max 2 weeks, maybe a bit longer. But NOT 5 months!
This is a rolling release, you HAVE to keep it updated.
As for emergency system. Take a look at https://www.ventoy.net/en/index.html
All you need is an iso file and a 8gb usb stick and you have a bootable version, the same way you install.
I am confused…
It seems to me that /boot partition is not the same as /boot/efi as shown in instillation guide:
I have zero clue what to either do with that picture or what you mean. IIRC you select “choose partition” (or something similar, NOT “partition manually”) in the installation and select partition 5.
That’s it.
I’m pretty sure calamares will figure everything out, or ask you. There is no need for selecting mountpoints if you do what has been instructed.
Edit
I think you select partition by clicking at the colored partition in the bottom, and there might also be a dropdown menu for boot, most likely already set to /dev/sda1.
I prefer emergency system on the same disk, where I have access in both directions sda2 ↔ sda5 and my preferred tools installed.
That’s what chroot is for.
But the user is the god of the system. 
(Ie you should do what you feel comfortable with)
There appears to be a bit of confusion here on account of what constitutes a boot partition. That which @bedna is referring to is the EFI system partition, which technically is indeed the partition from which you computer boots up, in that the EFI system partition will launch the EFI-compatible version of the GRUB boot loader.
What you @majo are talking about on the other hand is the /boot directory, which can indeed also be a mounpoint for a separate partition — as is the case on my own system, but I have my own reasons for that — but which normally is just part of the root filesystem.
And thus, if you have multiple distributions installed on the same drive, then each distribution will have its own /boot directory on its respective root filesystem itself, and the same EFI system partition mounted at /boot/efi in each of the distributions.
In a UEFI install, as is the case on your computer, the /boot directory contains the kernel images, the initcpios, and the GRUB configuration file. But it is the UEFI firmware which, by way of the EFI variables stored in the machine’s non-volatile memory, loads the boot loader from the EFI system partition. And it is the choice you make in the boot loader’s menu which determines which root filesystem must be booted from.
An EFI-capable GRUB always points at the root filesystem and the GRUB configuration file of the GRUB that was installed last, and therefore, you will want to have the GRUB of your main system be the one in charge. Hence the two commands I gave you.
By the way, @cscs, the reason why I’m not listing all the options in the grub-install command is that they are not needed. That particular system already has GRUB installed, and once the OP boots from that system, running grub-install will automatically detect all the correct options — that it must be an EFI install, that it must use /boot/efi, and so on. So all that’s really needed is the --recheck option. 
Figured as much but I wondered about the path. There being multiple grubs and all. Cheerios.
OK.
Thank you all for your help.
In weekend I will try to install the second system and will let you know.
Edit (as I can not add a new post):
Everything is I wanted.
I did as @Aragorn suggested and now the GRUB menu looks as below, with the main Manjaro on the top place and with a clear information about the second Manjaro on /dev/sda5.
What does not work on my machine (but it is not so important) is the option “UEFI Firmware Settings”, which gives an error message:
Error: Redirection to Firmware Configuration Is Not Supported by Current Firmware.
And probably from the same reason, on both system I obtain a different result of the command efibootmgr as other users are showing in other threads:
efibootmgr
BootCurrent: 0000
Timeout: 0 seconds
No BootOrder is set; firmware will attempt recovery
In the Pre UEFI civilisation (in MBR based systems) it was common practice to create a separate partition for /boot, and create a mountpoint to reference it, as you describe. This fell out of favour at some point when the need for a separate partition became widely disputed, and certain distributions began shipping with /boot as part of the root filesystem. Users today seem largely ignorant to this ever having been a practice.
In the Pre Warp (OS/2) civilisation, a boot partition might even have been on a separate removable disk (read: floppy).
For the benefit of passers-by, Is it good to make a separate partition for /boot (StackExchange) in both comments and answers, indicate many valid reasons for using a separate /boot partition (MBR).
Out with the old, in with the new:
UEFI systems today carry forward the same overall concept, albeit with new technology and naming conventions: The EFI System Partition (ESP) is on a separate partition; usually with a /boot/efi mountpoint residing under the root filesystem. I that note many users seem ignorant of this.
The full path to the Manjaro UEFI boot files on the ESP would then expand to something like /boot/efi/EFI/Manjaro or /boot/efi/EFI/BOOT in the case of the fallback bootloader.
In a UEFI-boot system, which yours is, all the bootloaders for all the distros you install will live/co-exist in the EFI System Partition, unless some distros also allow you (while installing) to place the bootloader of that distro on the root partition of that distro.
But that doesn’t mean the different distros write over the /boot/grub contents of other distros, since they each have their own /boot/grub on their own partitions.
But anyway let’s just focus on placing all the distros’ bootloaders on ESP. Then it’s a matter of you ranking the priority order of the different bootloaders and choosing which distro’s bootloader (and its grub menu) you want the machine to boot up when you turn it on.
You can usually change the order via your machines BIOS settings or from within any of your distros you can run the terminal command
efibootmgr
That command will show your the current order of the bootloaders. You can then change the order via terminal (read the last portion of the link above).
So after you install your 2nd Manjaro, you can check what the current EFI boot order is. If it’s still your original distro listed as first, then you need to boot into your first Manjaro install and follow Aragorn’s instructions on enabling os-prober and updating Manjaro#1’s grub so it recognises that there is another distro (Manjaro#2) on the system and will write that new entry into the boot menu.
I will also add that using rEFInd is a valid addition for any multiboot environment. This separate UEFI bootloader will allow you to choose which instance of Grub to boot, or even bypass Grub completely whenever desired. For the OP, this gives an easy solution to booting whichever Manjaro version he wishes, without needing to leave one instance Grubless.
sudo pacman -S refind
There is also the manjaro-refind-installer package which could be used instead, which I believe ensures consistent Manjaro theming, among other things.
pamac install manjaro-refind-installer
The only caveat is that some distributions (most notably Debian) tend to steal the default boot occasionally, when updated, which is easily corrected in BIOS by correcting the boot order.
However, to fix this occasional annoyance, there is a rEFInd command to move the rEFInd files to the fallback folder /EFI/BOOT, which then ensures that rEFInd is the first to load.
The simple procedure is outlined in the ArchWiki linked below.
I hope this is helpful.
rEFInd (ArchWiki)
about this command :
`sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck`,bootloader-id is the name that you will see on entry UEFI motherboard ,
if you have the SAME bootloader-id , it may update and show only one.
prefers add with another name to see both entry ( x2 for initrams and failback )
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