Try the following protocol:
ToSC v.5512
ToSC patch
http://www.interplay.com/support/bgtales/patch.html
directX8
http://www.microsoft.com/directx/homeuser/downloads
directX8 patch
http://www.interplay.com/support/bgtales/patch.html
no Roger Wilco
no firewall (either temporarily disable or configure ports as indicated)
You can MP with Dark Side installed but everyone playing has to have it. If only one or some has it this will cause problems.
I would recommend saving a new game at the point the main character appears in front of Winthorp's Inn. Then open that game in multiplayer; TCP/IP mode. When players join simply assign them slots in the arbitration screen. If everyone has the same setup above it should work fine.