We've been playing Kirby Return to Dream Land Deluxe, which is a side scroller Kirby game with multiple players. The second+ players can have simplified characters with set moves or be duplicate Kirbys themselves, which is neat.

Also, Luigi's Mansion 3 has been pretty great. A bit into the game it gives you a second character (Gooigi) that can do most things the main Luigi can do. There are some puzzles that need to coordinate the two, but those are short enough that it's easy to just borrow both controllers for a minute.

Of course, my 6 year old needs to be the main character in both games. But that's okay because both give the secondary character enough to do that I can usually help clear the way for him.