How to install dual Linux (different distros) in BTRFS with GRUB?


I would like to install two different linux distros, with dual boot. Each root filesystem will be in btrfs (with several subvolumes)
I am a bit lost because, i do not know if a have to have separate boot partitions or not.

Then, if i do not a have separate partitions, i will have one /boot folder in on partition, say sdb1 and another /boot folder in the other partition, say sdb2.

Which on would be active ? Which grub would be the one used by the system ?

Then, i also plan to take snapshots of each root filesystem and have them available within GRUB menus.

Currently, i have only one distro installed so that is fine, my snapshots are available in GRUB.
But, when i have two distros, how is it managed in grub ?
Does it mean that all snapshots from both distros will be mixed in grub submenus ?

Thanks for your help