You can do this just fine, but you need to keep the name of the folder as "Desktop" in your user directory, otherwise OS X won't recognize it. Basically, all you need to do is this:
(for the example, the partition with OS 9 on it is called OS9)
in the terminal, go to your home direcotry (type <font color="#0000ff"><b>cd</b></font>). Delete the old Desktop folder if it's there (<font color="#0000ff"><b>rm -rf Desktop</b></font> - make sure to save anything out of the old Desktop before you do this, otherwise you'll lose it all). Then type <font color="#0000ff"><b>ln -s /Volumes/OS9/Desktop\ Folder Desktop</b></font>
That's it, you'll be sharing the desktop folder with OS 9 after that. You may need to logout then log back in before the Finder notices the change.