This question is kind of related to this one.
Suppose someone makes a borei minei mezonos bracha on pasta (whose bracha achrona is al hamichya), and exempts rice (whose bracha achrona is borei nefashos) that he is planning on eating as well. If he eats only a small amount of pasta (not enough to warrant a bracha achrona), but a lot of rice, which bracha achrona should he say, or should he say both?