When I had Redhat 6.2 and Mandrake installed on one machine I basically did it the way trickykid described, but to save myself the hassle of sorting out two /boot directories (on two separate hard drives in my case), I just copied my RH kernel to the Mandrake /boot partition and had lilo look there for both. This worked because the two kernels had different names. I then added the appropriate stanzas to lilo.conf to like this:
I then saved the file and re-ran lilo, and everything worked perfectly. Each distro still had it's own /boot directory, but lilo only looked for the bootable kernels in Mandrake's /boot.
As far as editing fstab goes- it won't help you with the dual boot issue. But if you want to boot into one distro and have some of the other distro's partitions mount also, just create mount points for them and add a related line for each in /etc/fstab. For instance- if you're booting into Mandrake, but want the /home partition of Redhat to be available as well, create a /mnt/Redhat directory and then add a line similar to
mount /dev/hda5 /mnt/Redhat
to fstab. Obviously, the "hda5" is just an example; subsitute the actual partition of the Redhat /home directory instead.