It is weird, but the face of one person can resemble two very different faces at the same time. I once visited a class mate and was introduced to her mother. They looked remarkably alike. Then her father walked in the room and he was her spitting image as well. This was weird, because her father and mother didn't resemble one another at all! So from C looks like A and C looks like B, it does not necessarily follow that A looks like B.
Apparently it is possible that the determining features for one face sit e.g in the mouth and eyes and for another e.g in the eyebrows and nose. A third face can then combine all of these features and look like both (see illustration). When making portraits it is important to look for the determining features.