My thought is that it depends on the file system. If both operating systems use the same file system and one of them has dual boot capabilties, then it's possible. The only example I know of this was OS/2 and MS-DOS/Windows, if the partition was formatted as FAT16.
I'm not sure it would be possible with Unixes, as I don't know very much about the file systems concerned. I personally run Slackware, which is formatted as EXT2, and also WINXP, which uses NTFS formatting for its partition. Because the formatting of the file systems are different, it isn't possible, as far as I know, to install both of the operating systems on the same partition. And, even if it was, then you'd need a boot loader to switch between each of the operating systems. If one doesn't exist, then you'd have to write it yourself - and I *really* don't have a clue as to how to go about doing that.
If you're trying to install a second operating system without losing data from the first one, then maybe Gparted would be useful to you. It runs from a bootable CD, and enables you to resize a partition to free up disk space and to create new partitions outside the existing one. The homepage for it is here:
GParted -- LiveCD
Some caveats: back up your existing partition before you use Gparted. I used it with no problems, but it's always best to err on the safe side. And, if you're installing a form of Linux, you'll need *two* partitions, a bigger one for the Linux installation, and a smaller one to use as the swap partition.
HTH.
Install more than one OS in a single partition...