Cool. Here's how it works.
Fast user switching can be set up in the accounts control panel. Click Login Options and click Enable Fast User Switching. That should put an additional menu at right-hand side of the menu bar with your long account name on it. You can pull that menu down and select other users from it and login to their accounts while keeping yours open and running. A password is required to switch between accounts at all times.
With OS 10.3, you can run multiple instances of a program between users. If you don't know what that is, it means that you can run copies of the same program simultaneously with as many users as you want.
The important result of these situations is that you can host an TCP/IP game with one user and connect to it with the other, even if you can't usually host games. Once you've created the multiplayer game with your primary user, switch user accounts, open another instance of Diablo II, choose Other Multiplayer, then TCP/IP game, then the Join Game option, and enter an IP of 127.0.0.1 (Using this IP will keep your connection solely in your computer and won't require a bounceback from a router.)
Important note!
You need to set item permissions properly in order for this to work! When you create a character, the characters is only useable by the system user who created it. Whenever you make a character click your Diablo II Save folder and choose Get Info from the File menu. Then pick Ownership and Permissions and unlock it, then pick Read & Write from the Access dropdown menus and the Others drop down menu. Then, click the Apply to enclosed items... button to make all the files inside your Save folder readable and writeable by all users.
If you have any questions about this, let me know. I love helping out Mac folks, so I'd be glad to give you any more information that I can.