GRUB multiple boot partitions

Create /etc/grub.d/40_custom and replace 00000000-1111-2222-3333-444444444444 with second GRUB partition:

# A dummy entry to mark the separation
menuentry "Extracted from /dev/sdb2" {
        true
}

insmod part_msdos
insmod ext2
search --fs-uuid 00000000-1111-2222-3333-444444444444 --set othergrub2 --no-floppy
extract_entries_source ($othergrub2)/boot/grub/grub.cfg