In the main lobby, press  and send an invite to a friend or boosting partner. Make sure that you have each other as friends on your PSN account friendslist, as the trophy might not pop otherwise.

Once either player has joined the other players lobby (indicated by one of the + signs in the boxes being occupied by the level icon of the player that joined, in the upper part of the HUD), head into a match and simply play it. Whether the match results in a win or loss does not matter. Heading into a match is explained in  Victory.