Originally posted by Detrius:
You mean Mac OS X Server and Mac OS X Client? It's not possible. You can however have classic installed on a partition with either of those.
Classic's directory structure was altered so it can coexist with OS X. You can even install classic on a disk image. Technically, you could put OS X on a disk image, but you cannot boot from it.
OS X and OS X server have pretty much identical directory structures. So you would delete some old stuff while installing the second OS.
In any case, it's not a good idea to put two OSes on one partition, even if you can.