The second player is not visible on the screen but is essential to building the tower because no player has access to all the colors necessary. This also means that it is indeed possible to not being able to finish the game because the patterns are created by accident.
The fifth game by Jason Rohrer is once again very experimental. It is only playable with two players and the goal is to build a tower based on a pre-defined pattern using and combining building blocks of different colours. But since you can't see the other player and their blocks the resulting pattern may not be what you wished for.
This page contains co-op info for Between on the PC. All information about Between was correct at the time of posting. Information is subject to change. If you see any errors please email us.