Why are there no recipes combining both yeast and baking powder?
I've never seen anything with both yeast and baking powder. What's the reason for that? Shouldn't e.g. muffins get even more fluffy by adding some yeast? Or bread by adding baking powder? I know there are differences in flavor, but there are cakes with yeast as also non-sweet things with baking powder. So that shouldn't be the problem...
Actually, there is one baked good combining yeast and baking powder (or sometimes also just bicarbonate of soda): Crumpets, see, e.g., http://en.wikipedia.org/wiki/Crumpet for a general description, and here http://britishfood.about.com/od/eorecipes/r/Recipe-Traditional-English-Crumpets.htm and here http://www.bbc.co.uk/food/recipes/crumpets_61013 are two recipes.
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.
This is a really critical observation, which addresses a lot of lore about leaveners. I've definitely heard it said that you should just add a little more baking powder/soda to get a fluffier result in many recipes, or (in a similar way) that old baking powder can just be used if you use more or even double of it. (The assumption being that extra leavener can't hurt if the baking powder hasn't lost much strength). It's a point that I've rarely seen addressed in cookbooks, and I only figured it out myself some years ago through trial and error.