Well it depends on how big your snake is, but I would try offering the extra fuzzie just like you normally would, after he gets the first one completely down. He will let you know if he is hungry enough to eat 2 fuzzies
Also I think it is easier for them to digest 2 smaller prey items than one big one, so that would probably be the way I'd go. Depending on his size a grown mouse might be too big anyway (aim for something not too much bigger than your snake is at it's widest point).
Hope that helps