One loud, one plain
Pick one face with a personality and let the other one do the reading. Two loud faces fight on the page the same way two strong sauces fight on a plate.
Match the size of the x
Put the two faces side by side at the same point size. If the lowercase letters are close in height, the pair will sit happily on one line. If not, one of them will always look a size too small.
Taste it at the real size
A pairing that looks great on a 72 point poster can fall apart on a 9 point label. We print every pairing at its smallest size before we show it to a client. It saves a lot of awkward second rounds.