ye so it should be showing the keys to press when you walk in front of the door! it's right shift and 6.
The controls for two players one keyboard are unfortunately really weird because we have a lot of holding down keys in our game, and most keyboards have a really hard time reading a bunch of inputs at once, so the 2 player one keyboard keys are made to avoid that problem as much as possible. Unfortunately that makes for some pretty unintuitive controls, i'd highly recommend playing with a controller for one player if you at all have the option.
The whole 2p1k controls, if this helps, are:
player 1: wasd, 456, space
player 2: arrows, ./rshift, enter