If I eat less than a k’zayis of cake; and less than a k’zayis of grapes; do I make a brocha Achrona? And if I do; is it Borei Nefushois, or Ahl Hamichya V’ahl hu’aitz?
[What is the blessing if] you ate a half-kezayit of crackers and a half-kezayit of apple? The proper bracha achrona is Borei Nefashot. The reason is because the apple cannot "go up" in status to combine and obligate Al HaMichya, but the cracker can "go down" to obligate a Borei Nefashot.
This is also the ruling of Kitzur Shulchan Aruch (51:4)
So, if you ate half a kazayis of something that requires the after-berachah, Borei nefashos rabbos, [and then you ate] another half a kazayis of a food after which the "three-faceted" [after-] berachah, is said or even if you ate half a kazayis of bread, you recite the after-berachah, Borei nefashos rabbos. And it seems to me that the same ruling applies if you ate half a kazayis of fruit after which the berachah Al ha'eitz is said, together with half a kazayis of food after which the berachah Al hamichyah is said.
This site mentions R Shlomo Zalman Auerbach disagrees and recommends "al hamichiya v'al haeitz" but I haven't yet seen it "inside".