Yay, more PSP hacks. EsNetsc sent word that PSPVault has a story about playing two player Ad-Hoc games with one UMD game disc. It seems to (at the least) work with Tony Hawk Underground and Ridge Racer. You’ll need 2 PSPs, 2 people and one copy of the game. Complete instructions here and here.
Play 2 player PSP games with one game