What you do is, you and a friend are already in a private match, Oldschool, and you invite the person on ur frinds list, that is already in the game with you and an error box should come up "should say friend is already in gmae with you",
hold the ps button down, til black screen comes up, which gives u the option to turn off system or turn off controller, pick the turn off contoller option, after its off, turn it back on and go back into the game,
jump in the air and hit select, a black screen should come up with an error, then let ur friend knife you in the butt 20 to 30 times, hit select again and you should be up high in the air, do this at places where you want to get on top of etc.. buildings, Hope this helps
Edit: If I have not explained this right, please fill in the gaps, Thx