Has anyone considered that there may be no difference between the two except environment? If someone sleeps with an outsider, what pops out beautywise determines what it "is". If she has metallic hair, keen eyes, and a halo, she's an assimar. A pair of horns and three breasts? Tiefling.
Which brings up the potential of a pair of tiefling/assaimar twins.
Another similar idea is that it's the plane itself that "creates" tanaari, devas etc. A deva stuck on the Abyss for a few decades would "degenerate" into something vile. Which is one reason there is not long term massive attacks from Aborea into Gehenna. The Blood war is easily explainable. There really isn't THAT MUCH DIFFERENCE between the two. Sure, they had to make them so it jived with their alignment system, but really!
I think there's more to a tiefling or aasimar than upbringing. There are specific differences between the two, not just a label to roughly categorize planetouched offspring into two camps based on appearance.
Therefore, I couldn't see much possibility of twins in which one is an aasimar and one a tiefling.
EXCEPT
In the case of fraternal twins (probably not as exciting because it doesn't have that mystical feel that identical twins have), the genetic material is no more linked than that of any sibling. So, in a family in which the parents have a mix of planar creature in their heritage, you could get something like this. If three generations ago you have a marilith in your family on one side and then four generations ago you have a ghaele on the other side, there's no telling what your kids might be. You could have a tiefling for a kid, maybe, or maybe an aasimar. Or maybe you have a little tiefling boy and then later a little aasimar girl. Or maybe, in even rarer circumstances, have fraternal twins and one is each.
Of course, this is all quite unusual. Even with the necessary outsider blood in your family, there's no telling when it will manifest in a planetouched child. Certainly not everyone in the family shows the signs. I guess it just takes the right little bits of planetouched chromosomes to converge.