Considering the extreme degradation that can be found in the bottom of JAV swamp, I don't think rectal stretchibility is the limiting factor here. I think the reason you don't see them much is exactly that not many (male) fans are demanding to see double anal.
Speaking for myself, I love anal sex. But I don't want to see certain things in anal porn: chocolate chips, double anal, or gaping lasting more than 1.25 seconds.
I agree that hentai fans love double anal. I don't agree we could extend that to say AV fans are very interested in double anal. There are a lot themes and memes that are essential in hentai universe that just don't work in AV fandom.
Take a fairly mild example, double oral (?) is very common in hental, a girl (or any kind of feminine lifeform) sucking two cocks at once. Just like double anal, you also (almost) never see double oral. We can be pretty sure there's no shortage of male and female performers willing to perform such an act if studio/director ask for it. But we don't see it (much) so it must be AV fans don't demand it.