you have 2 players who are both in custom mutations on zombies.
Player 1 needs to use there D-Pad to move it to select/hover above edit game rules then join player 2 who is also in custom mutations zombies.
Player 1 needs to turn controller off, Player 2 needs to leave party and bring party. Player 1 can then press x or a to get rid of the connection lost to controller message.
Player 2 then needs to search for a RUSH public match.
Now its best if you find a empty lobby, if you get in to a empty lobby have player 2 back out alone.
Player 1 now needs to spam x or a to get the edit screen up as player 2 leaves.
Edit the rules to how you want them and have friends quickly join back if needs be.