It's extremely rare, but as others have said it can happen. It's called diphallia and you can read about it here:
Diphallia - Wikipedia, the free encyclopedia
It certainly is not a trait that will spread to the general population since it is extremely rare but almost always results in infertility (and who knows, maybe the few women claiming that they were impregnated by a diphallic man were impregnated by someone else.)
The two penises are usually side-by-side, so one atop the other is very very rare. One penis is almost always a lot smaller than the other, which is a blessing really. I know if I had two dicks I'd definitely love an extra skinny one for anal, or just a tiny extra to DP a vagina with after we're both worked up.
Diphallia (without the unpleasant things that go with it) is usually my answer to the question 'if you could have any superpower, which would you choose?'
ps the girl in that vid is really cute. A soft body in a thong may be my greatest weakness.