Why are there no recipes combining both yeast and baking powder?
The existing answers already explain why yeast and baking powder won't work together. But even if they did, you wouldn't have a reason to use them. 

You seem to think that fluffiness depends on the amount of gas produced by the leaveners. In fact, it depends on both the gas and the ability of the dough to trap that gas. If you produce too much gas (no matter whether through yeast or through baking powder), then the fluffiness will be less than when using the optimal amount of leavener. This happens because your dough cannot hold the gas and the bubbles break, resulting in the dough deflating like a punctured tire. So even if combining two leaveners would have resulted in more "blowing big" action (which it doesn't, see the other answers), you would not end up with a fluffier end product. If you want fluffy muffins, you have to use a recipe and a technique which is best capable of retaining the gas produced by the baking powder. The amount of gas production is not a bottleneck.