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.
