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?
This question is kind of related to this one.
In the Siddur of the Baa' Hatanya in Seder Birkas Hanehenin chapter 8:7 he writes: All foods combine to make up a Kezayis to say the after Beracha of the lower level amongst them for example if one ate half a Kezayis of the 7 species and half a Kezayis of something which one says Borei Nefoshos - one says Borei Nefoshos.
If one ate half a Kezayis of bread and half a Kezayis of a Kezayis of the 7 species - one says Me'ain Sholosh.
All liquids combine with each other as well. But foods and liquids do not combine neither to a Kezayis nor to a Re'vi'is.
See also the very end of chapter 1.
