haven't got this myself yet but this is off the bethesda blog -
Is there any co-op gameplay included with DOOM 3 BFG Edition?
The co-op gameplay for DOOM 3 was only included with the original Xbox release of DOOM 3. As BFG Edition is based upon the original PC release, there is no online/offline cooperative play. The Xbox 360 and PS3 versions of DOOM and DOOM II feature online and splitscreen co-op supporting 2-4 players.