At the moment, this game is only restricted to it's Single Player capabilities. The fans have wanted a co-op feature and therefore the fans had therefore started working on a multiplayer mod, and which one works but it is only restricted to local controls on a single PC. It does need an application for said mod to be able to be used, and this application is called Cemu and requires to be open in two clients at once.
Cemu is a Wii-U Emulator used on a PC. This mod is extremely in it's early stages of development, so if you do actually try it, don't expect a lot of features.