The player-made "BioShock Together" mod allows two players to cooperate through the BioShock campaign. Developed by independent creator VoidCrownStudios, it has been released on Nexus Mods and is currently in an early Alpha testing phase.
According to the mod description, two players can fight enemies, hack machines, collect story mission items, and progress through the storyline together in Rapture. Players can also individually decide how to treat Little Sisters. The mod also provides partner location markers and handles synchronization pauses when players explore different areas.
The mod author stated that they have played through the entire campaign from start to finish using this mod, provided character models and scene interactions are synchronized. The mod supports LAN and online hosting; the claim of a full playthrough comes from the author themselves and should be viewed in light of its current early testing status.
When using the mod, both players need to own the Windows version of BioShock on Steam, install the same version of the mod, and select the same difficulty at the start. Neither player should have other mods installed or enabled. This is an early test mod created by players, not an officially added co-op mode for the game.