Don't remember where to get it.. MAYBE hellmonkeys site? (not sure if it still exists havent checked in a long time) but i believe it lets u replace ur current CD key with another one
But would i need to have the games installed on two different partitions or could i run off the same diablo2lod.exe with different cds. I am not sure what the loading program is called, i think it is diablo2lod.exe
When I did it I just coppied my D2 folder into another folder(meaning I had the game in two different folders) then I used the CDkey refiller(or edited registry) to change one of the client's CDkeys, put shortcuts for both of them on the desktop, made them both run in window mode. Tada.