There may be another way to do this cheat, but I like the funny result this one gets. At some point in the game, there should be a trouble in the trouble centre saying 'I wanna meet Luigi!' Take on this trouble, and go to Poshley Heights as normal. Now, use your Luigi Emblem (this can be found in the Poshley Sanctum) and talk to the toad near the fountain. She will go all crazy, saying "Yay! Luigi came! Luigi came!" But then, the REAL Luigi will appear, saying 'Hi Bro. Say, why are you dressed like me?' The ending result is very humorous. Try it out!