In my campaign, Tiefling doesn't work as a race quite the same way. Instead, if someone makes a certain sort of pact with infernal beings, that person's bloodline now carries a hidden taint. At any point (even dozens of generations later), that taint might suddenly erupt to make some baby be born a Tiefling instead of a human. (I haven't yet had to make a ruling on if this can happen for non-humans, but since humans are supposed to be the corruptible race, for the moment, it's just human bloodlines.)
So it's perfectly possible to have twins, and one is a tiefling and the other is a human.
I've also ruled that tiefling parents breeding with humans are 50% likely to produce a tiefling; two tiefling parents should always produce a tiefling.
So it's perfectly possible to have twins, and one is a tiefling and the other is a human.
I've also ruled that tiefling parents breeding with humans are 50% likely to produce a tiefling; two tiefling parents should always produce a tiefling.