They will control Player 1 with the keyboard and you will continue on as Player 2 using the controller. Once you’re on the map, press A on the controller a few times to join in as Player 2. 2- (And probably the best option) Using.
its beta but I already play it this way, and it works pretty well (even considering that my connection is really bad where I live). 1- Using Nvidia Gamestream co-op (in case you have an Nvidia graphics card compatible with this feature). Load into the game using only the keyboard. Actually, theres (at least) two ways to enjoy this game online. As the game host, Launch the game with your controller already plugged in. In the Nvidia app under General settings select “Enable experimental features” to get the update and also turn on In-Game Overlay, and on Gamestream feature’s Stream settings say “Yes” to allowing a friend to join a game and play as a guest. In the meantime, here’s a video guide on how PC players with the Nvidia Experience desktop app can play Cuphead together online.
How To Play Cuphead Online Co-op Multiplayer on PC: While both the Xbox One and PC versions of Cuphead have Local Co-op Multiplayer, the developers have yet to add an Online Co-op Multiplayer option, although one was hinted as part of a possible future game update!