It's funny, because I was just having this conversation / argument with some friends a week or so ago. One friend insisted that he'd done it, but I said pics or it didn't happen.
My husband and I went home and did some googling and came across this thread
with some links to some transexual porn with simultaneous anal - but it's the same couple in all the videos!
So clearly it *can* be done! My husband and I were not brave or flexible enough to try it, but maybe someday...