Whatever you want
What's important is that you don't install Mandy and RH to the same partitions. You can share certain "directories" assuming they are on seperate partitions, but this is REALLY not necessary. Some will say share /boot and maybe /home but you do NOT need to do that.
IF you do use seperate /boot directories though, you will need to copy the kernel of either distro to the /boot directory of the distro with which you will use your boot manager.
I would suggest using lilo, just because I do and have no problems. I can also help you trouble shoot better with that, but that's not to say there aren't TONS of grub users on here, that could probably also easily help you with that if that's your boot man of choice.
So actually, yes you will want to make AT LEAST 1 partition to install RH to, to directly answer your Question
Cool