You need separate partitions for the root file system. ie if you have Crunch on sda1 then put buntu on sda2, or what ever the next partition will be on the system.
Your /home partition may be ( or may not be ) safe to share. A lot depends on the desktops you plan on running on the systems. If they are the same desktop, sharing is possible. Watch the levels of each, you may cause problems from one to the other. It is safer to have separate /home partitions.
Swap can safely be shared.
Boot manager. Only one installation can manage your boot manager. I know buntu uses grub 2. Check into Crunch. Pick one, and only one to manage the booting of the system.
If you separate the partitions, and understand which one system manages the booting, you should be O.K.