As you did not mention if Manjaro and its bootloader is installed on sda or sdb, this is hard to say. If Manjaro was installed on sda, just make sure your firmware boots into sda where Manjaro’s grub is installed.
Why did I never think about this? I’ve just changed the boot order and took the sda to the first place and it works.
A note to the readers
While this is the solution to fire up GRUB on boot, Windows is not shown as an entry since the two systems are on different physical devices. I don’t know how to solve this, probably it can be solved with configuring GRUB manually or GRUB Customizer. However, as Manjaro is my primary environment, I don’t have any problem directly booting it up.
To switch to Windows on bootup, each time you have to trigger boot selector (usually by tapping F8, F2 or DEL continuously) and select the other device.
You need to enable OS Prober in GRUB config file (GRUB_DISABLE_OS_PROBER=false), then update grub (sudo update-grub) for the menu entries to get updated. If it doesn’t detect Windows then your OS are not installed the same way (one MBR the other EFI), for OS Prober to detect the second OS, both OS need to be installed the same way.