There are emulators out there like Kega Fusion (Sega emulator):
http://www.eidolons-inn.net/tiki-index.php?page=Kega
that lets you play ANY game which originally has multiplayer (2 or more players) REMOTELY, so it is not necessary to all players be on the same PC using the same keyboard. So one PC act as a server (one player), the others as clients (other players). So I think each player send his keystrokes to his local game and to the server, and the server send these to the other clients, and each client sent these to his local game, something like that I suppose...
Try Kega Fusion with any multiplayer game by internet, it works well! So... THIS CAN BE ALSO DONE TECHNICALLY IN DOSBOX. I don't know If I have explained this well...

Could this be possible to implement in Dosbox? Try Fusion to see this