From the main screen, go to "Multiplayer Lobby", and then choose "Xbox Live". Then switch the boxes to read "Ranked Match" and "LOG's Lost Challenges".
Have your partner do the same.
Then start searching. It will eventually find that one partner and say "Opponent 1" and it will say that it is waiting for 6 more players. Just wait about 5 minutes or less and it will eventually start the match up with just you two.
Then just win.