The latest update on the Windows version of Borderlands 2 and Borderlands the Pre-Sequel has temporarily disabled cross-play between Windows, Mac, and Linux.
Currently, Mac and Linux players are able to play together, but are unable to connect with Windows players.
We are working on an update to bring Borderlands 2 back in sync, but we do not currently have an ETA for release. We encourage players to join our #Borderlands Discord Channel to coordinate co-op matches with Mac and Linux players, as we work towards delivering the update to re-enable cross-platform play with Windows players.
Please know this is temporary, and soon, you’ll be playing together again on the latest version of Borderlands 2 and Borderlands The Pre-Sequel