To get a mushroom house to appear in multiplayer, you simply need to clear the level when the time has the same 2 end digits next to each other when you touch the flag pole. Examples: 544, 322, 411. Depending on the two end digits, you will make different mushroom houses appear at the start point, as you would in single player when you save a Toad, only the digits are mostly red mushroom houses. Two zeros at the end will get you nothing. When you do this, you hear the original Super Mario Bros. level clear music and see a number of fireworks depending on what the last 2 digits are. 1 firework for 11, 2 for 22, and so on.