Rain world (with the downpour dlc) (incredible game)
Just shapes and beats
Crawl
Binding of Isaac (pretty sure some dlc is required)
Snipperclips plus
Duck game
Pikuniku
A hat in time (I haven't actually played myself but my friends have)
There's also:
Battleblock theater
Towerfall
Samurai gunn
Distance
Human fall flat
Rocket league
Ultimate chicken horse
Enter the gungeon
Almost any fighting game
You can also still play a lot of old couch co-op games through an emulator or get couch co-op mods for newer games (another commenter mentioned Minecraft). There's a lot of options.
it takes two was pretty good, also I forgot if portal 2 multiplayer has split screen or not but that would be fun too
edit: you can also mod Minecraft java edition to have split screen and controller support, haven't personally tried it, but I saw a YouTuber do it lol (iirc bedrock edition has it built in but bedrock requires windows and is worse imo)
edit 2: I forgot ibb and obb. Its good, but was too difficult to play with my (at the time) 10yr sibling. Not sure how it compares with adults.