i tried nucleus coop the other day but it crashed when clicking play on the app, got no help from discord channel or reddit sub so i tried universal splitscreen and that worked great, so i think if you have issues with nucleus coop (it's pretty buggy), try universal split screen instead, the great thing about that is it doesn't do the hacky stuff nucleus does like creating symlinks (creates a copy of the game folder for each player).
universal splitscreen just allows you to assign controllers to each player and also has a handle close option to close the open handle in the game exe that stops a second instance of the game launching. which is for anyone who doesn't want to search for it
you just have to paste that in the handle box.
you have to create a 2nd live account for player 2 and you need the steam overlay on (i think), you then go to the xbox site and sign in one profile, search and add the 2nd gamertag, then log out and sign in to the first again and confirm the friend. Then you can find the friend on the mcc and invite.
All you need to do is open up two game instances, use the app to assign a controller to one window and then another to the other window, resize them and boom....splitscreen.
will do me fine till they add native splitscreen...(when/if that ever happens)