You can have both on the same volume. You can also carry a 9-volt battery in a pocket full of change. Maybe it'll fall just right, short and burn your leg…or maybe it won't. Having two different OS's parked in the same place is similar. Depending on what you do with them and how you do it, maybe everything will be just fine or maybe stuff will crash and burn every other time you boot up.
It has mostly to do with the fact that they use different file systems and directories. With only one volume, it's possible for OS9 to write something in the directory area that OSX can't understand-or vice versa. So, next time you boot OSX, it automatically runs disk utility to "fix" what it doesn't like. Then you go back and boot OS9 and now it has a problem so it re-fixes the directory and on and on and on. Very soon it's an unrecoverable mess and you're having to completely erase the drive and start over.
My advice is: Partition the drive. It's no big deal to do. Then:
Go to Tiger Spotlight prefs and prevent Spotlight from indexing the OS9 partition.
Or, just don't bother, as I said, maybe you'll get lucky - at least for a while, but when shit starts going wrong and you start to get kernel panics and crashes and freezes, you'll remember this little warning…