Which games can be splitscreened using Nucleus Co-Op?
There are a lot of supported games, the full list of supported games can be found here(reddit), or here(the hub).
A ton of games are now supported thanks to the amazing program called Goldberg Emulator, developed by Mr. Goldberg, a big thank you to him.
Read the Goldberg FAQ if you want to know more.
Do you not see the game you want to play in the list of supported games? Request it in the comment section here, or check out this guide on how to create a handler for that game yourself. For more info on handlers, see what are handlers?.