Whichever one entered the battlefield most recently "wins." The controller of that Graveyard Busybody has all the graveyards, and any other player's Graveyard Busybody is a 0/0 that is put into the graveyard that player used to have as a state-based action
The Commander format is all about creativity, strategy, and unforgettable plays, and the decks were about to dive into capture that spirit perfectly
Manifold Mouse is a way to consistently boost up your attacking mice by giving one of them your choice of trample or double strike
They come into play tapped, but pairing this with some kind of mass land destruction leaves you with everything you had and your opponents with nothing
The most important trigger in the card, however, is the ability to untap up to three lands
Can you really run any number of Relentless Rats and Rat Colony