Just spent a few minutes trying to make this work. I don’t have 2 copies of D2R so someone would have to confirm they can actually join games.
- Download Process explorer
- Launch D2R from Bnet
- Open Process Explorer
- Find D2R on the left side, click on it
- Find the Event "Sessions\1\BaseNamedObjects\Diablo II Check For Other Instances
- Right click the event and select ‘Close Handle’
- Open another D2R instance using the .exe (not Bnet)
- Close process explorer (you dont have to, but it eats moderate resources).