I've gotten singles, twins, AND triplets when both of the (same) parents had the fertility reward, it's still random, but it looks like the chance of multiples increases for each parent that has it.   I've also bred several different pregnancies trying out the apples and watermelons theory, and I've had several boys on a watermelon only diet, as well as girls on the apple diet, and vice versa.

I have, however, never had a mixed gender multiple birth, and I'm pretty sure that's a bug that EAxis didn't fix before release.