The most famous instance of a Metro 2033 co-op mod came from a modder (often cited as active on ModDB or Russian modding forums) who released videos showing two players running through the game together.
While not traditional co-op, significant modding efforts are expanding the base game: metro 2033 coop mod
For those who want "co-op" strategy, this mod lets you control factions in the Moscow Metro together. Why a True Mod Doesn't Exist Yet The most famous instance of a Metro 2033
While the Metro 2033 Coop mod is an impressive achievement, it's not without its challenges and limitations: metro 2033 coop mod
Since you didn't specify which game in the series you are talking about, I have broken this down into the two most likely scenarios.