I've experienced double anal pnetration oonly once. One of the guys was above average and the other was average. It was fun and there was no pain involved, just a different sensation which is difficult to describe. As much as I love anal sex, I have to be very comfortable with a man to let him inside of my body. It all has to do with what is going on in my mind and how much my brain is "turned on". To find 2 guys I feel that comfortable with at the same time is rare for me. So it's only happened once.
I have also had the experience of going limp while with a well endowed stud, and it had nothing to do with the size, or pain, or that I was turned off in some way. It was actually most pleasurable and I still use that experience as masturbation material to this day (it happened more than 10 years ago). The top was very concerned about my limpness and it took some discussion to convince him that he wasn't killing me, it felt amazing even though I was soft, and that he needed to just keep on pumping. His dick was the equivalent of 2 beer cans stacked on one another. All I can remember was that I felt a non-painful pressure when his stroke went ball deep, and I lost my erection. All of the good sensations were still there. In the end we were both very satisfied. I just couldn't cum until after he was finished.