You couldn't install two different distros on one partition by definition, because (I'm almost 100% sure) that a partition can only be mounted as one filesystem, eg as /
Even if you made directories such as /mdk and /rh I think you'd have to hack every package to make every program work in the correct fake root directory instead of the real root directory.
I really doubt you could mount something like /dev/hda5 as / and then /mdk as /
Just cut the big partition in two.
You can copy each install's kernel images etc into each's /boot directory to make configuring your boot loader slightly easier, if you find it too hard to add /mnt/hda6/boot instead of /boot for a few entries