I have a multiboot setup, and add/remove distros often enough that a
having a boot partition is a must for me. The boot partition's
grub.cfg file just chainloads each distro's grub2 (located in each
distro's partition; each distro has its own /boot). The boot
partition's grub.cfg is updated manually when I install a new distro.
My preferred setup would be a single boot partition with the necessary
parts each distro's /boot automagically installed into a separate
directory within the boot partition.