This still works & I keep getting messages about it. So here I'll try to explain.
1) Start Mwr multiplayer
2) Log in with 2nd controller
3) Start Cage Match & back out really fast 3 times
4) With your 2nd controller push x or a to play while at the same time find a cage match with your main controller
5) If Cage match is locked or greyed out you were too slow
6) To retry just back out to single player then go back to multiplayer & repeat #'s 3&4
7) If you are doing it right and quick enough you will notice that when you go to find cage match for the last time it will be unlocked but only for a brief second & it will lock as soon as it notices you are playing split screen.
You will need to find that last cage match before it locks the playlist selection.
9) If it loads the 2nd controllers profile before you search for the last cage match it will not work
10) You must search for the last cage match while at the same time your 2nd controllers profile is loading
11) If done right when you search for cage match the last time you will get "please wait" then it will put you into a pre game lobby with your 2nd controller.
12) If you finally get it you better not back out of that lobby or you will have to try a thousand more times to get back in it.
Hope this helps, good luck!