tziruf of different grains for a k'zayis of matzah

I imagine yesterday was cleaning day for almost everyone spending Pesach at home.... I am aching from scrubbing, but I enjoy spending Pesach at home so much that it is well worth it.

The Minchas Chinuch debates whether one can combine eating part of a k’zayis made from one type of grain with part of a k’zayis made from a different type of grain to fulfill the mitzvah of matzah. Since the mitzvah can be fulfilled with any of five different types of grain, why would one think that one cannot combine different pieces together to get to the total of a k’zayis? In fact, the halacha is clear with respect to challah that breads made of different grains may be combined together to add up to the total shiur to be chayav in challah.

Perhaps the issue depends on how one understands the halacha of tziruf, combining items together. Does tziruf mean that items which may appear to be different are in fact part of the same family and therefore can be combined, or does tziruf mean that items can be combined in certain contexts despite the fact that they are inherently different? In other words, is tziruf a siman that items are the same, or a sibah, a new halacha that allows for combining different elements? If tziruf is a siman, a giluy milsa that items belong to the same family, what is true for challah should also be true for matzah. But if tziruf is a new din, then perhaps it is true only in some contexts and not others. Since there is no explicit source that tells us such a din exists by matzah (see the Minchas Chinuch’s attempts to find one), there is room to argue that no such option exists.