Regrettably, the format of Entourage:2001 and Entourage:X databases are different, so you can't have them use the same files. While Entourage X can import the Entourage:2001 data, Microsoft hasn't provided a path to move data the other way (not unreasonable.) There is a set of Export/Import AppleScripts from Paul Berkowitz which you can find at AppleScript Central or his home page which can move things the other way, but that isn't very convenient for frequent changes.

Back when I was switching frequently between OS 9 and X, I just stuck with Enoturage:2001, using it under OS 9 and under OS X (via Classic.) That way I had all my mail accessible in either OS.

The only other way I can think of is to use IMAP (or Hotmail) mail accounts, whihch keep your mail on the server rather than on your Mac, making it accessible from either program. But that doesn't solve the address book or calendar, adn not all mail providers offer an IMAP option.