If one ate cake and rice (a kezayis of each), what bracha achrona should he say? Only al hamichya, since that would cover the rice as well?
This is a very good question. The Sharei Teshuva 208-5 brings Mahari Mocho that if one mistakenly said al Hamichya on rice he doesn’t say borei nefashosh afterwards. The question then remains can this be done L’chatchila ( this is your question)? It is controversial. According to some poskim, the Kaf Hachaim 208-41, Ben Ish Chai Pinchas-18, say that it may be done. However the Shevet Halevi vol.9-65 disagrees, he holds that one should not have the rice in mind when saying al Hamichya and then he may say borei nefashosh.
There are two other ways to get around this issue. Either by making the borie nefashos before the al hamichya, or if he ate or drank something else he will have to say borie nefashos anyways.
See above sources, Sharei Habracha chap 11 ftnt 79, piskei Teshuvos 208-13