this is not mine but i found this when none of the others worked for me.
it works perfectly!
+ rep if it helped you out please or atleast comment and thank.


TEXT TUTORAL BELOW - (P1) = Player 1 (P2) = Player 2
Text Tutorial:
P1/2: Sign in a Guest.
P1: Load up Nuketown/Gametype through Player Channel.
P1: Search for Combat Training - Objective 3 Times
P1: Highlight Public Match, Hit "Y", and Highlight your friends name
P2: Search Combat Training - Objective Twice.
P2: Load up Nuketown/Gametype through Player Channel.
P2: Click "B" and Highlight "Yes" to leave game.
P1: Join P2.
P2: Leave as soon as P1 joins.
P1: You now have 3 seconds to go to your player channel and Highlight the Gametype.
P1: Click "A" on Gametype. Highlight "Yes".
P2: Reload Gametype through Player Channel.
P2: Join P1.
P1: Hit "Yes" as soon as P2 Joins.
P1: Join P2.
Done!