63b Rashi explains that Rekikin and Challos are grinded down to a flour state and mixed together (gmara-uballalan). only after that step, the gmara says kmitza is done. Such kmitza of the mixed flour will certainly contain flour of both Rekikin and Challos. How then can the next line of gmara state that if only Rekik OR Challah ended up in the kmitza he's yotzeih? how is this scientifically possible when pulling the kmitza from the mixed flour (Rashi)!?
Daniel Gray, TORONTO Canada
It seems that this means that he did not mix in the Rekikin and Chalos as thoroughly as he should have. Even so, this is not Me'akev b'Di'eved. Indeed, if he would have mixed them properly it would not be possible only to take Rekikin or Chalos from the mixture; but if he mixed them only partially, in a way that made it still possible to take one of the Rekikin or Chalos without taking something from both, he is still Yotzei.
It is possible to make a slight addition to the above based on what the Hebrew ArtScroll edition cites (63b, note 3) in the name of the Chafetz Chayim's commentary to Toras Kohanim. He writes that since the Minchah is composed of both Rekikin and Chalos, even though the actual Kometz was only from one of these sorts, this is sufficient b'Di'eved. The ingredients are Me'akev b'Di'eved, but the mixing is not Me'akev b'Di'eved.
Dovid Bloom