you do a speedrun of the single player part but you alternate between players throughout the different chapters. same rules that apply to the normal single player speedrun but the 2 players split the chapters 4:5 and they can do what ever ones they want. so its basically portal 2 single player any% multiplayer

If you can get like 3 runs tell a moderator and they can create the category, if you cannot get 3 different runs I wouldn't bother as the category would be dead.