The primary reasons for this lack of a "Skyrim Together" style mod are technical:
Surviving the Dark Together: The Quest for a Metro 2033 Co-Op Mod metro 2033 co-op mod
As of , here is the comprehensive status of co-op mods and official multiplayer efforts for the game that started it all. The Current Reality: Is There a Co-Op Mod? The primary reasons for this lack of a
Technically, there is that allows you to play the full campaign of Metro 2033 or its Redux version with a friend. metro 2033 co-op mod