Cuphead is a brutal, unforgiving game. Fortunately, it does have the ability to share that pain with a friend in co-op. When playing in multiplayer, your friend gets to take control of Mugman, Cuphead’s sibling. Because of the drop-in nature of it all, player two will also start the game at whatever point they decide to join with the amount of coins you have collected as Cuphead up until that point. In other words, make sure you take your friend shopping before you try to take out that boss you are stuck on, eh?
To get started in Cuphead co-op, simply connect two controllers, load up the game, and press any button on the second controller to have Mugman appear. You can also play with a keyboard and one controller if you wish, though sometimes you have to disconnect the controller in the menu for it to register.
Currently there is no online co-op functionality, but the developers are keen to add it at some point in the future. There are tutorials online to perform a workaround and have it running in online co-op with help from Nvidia’s GeForce Experience. That method is unofficial, of course, so only do it if you really must play with a friend who can’t come round for some couch co-op.