You could put the two OS's on different partitions on the same drive but you would not be able to run them at the same time.
Having said that, I wonder if it would be possible with something like Parallels, kinda run a Virtual Machine.
Wonder why you would want to.
I was wondering if it is possible to run tiger while booted into leopard like how you can run windows inside of mac osx. (Not the way bootcamp works)