I've done DP a few times as a bottom. It takes alot skill, patience, and personally I think it takes a well trained bottom to really make it work. I've been really well trained on HUGE cock (esp. thickness) and usually have no trouble at all taking whatever pokes through the back door, but a few years ago a friend tried it on me for the first time and it was amazing!
Mainly because I was really surprised it didn't hurt as much as I figured, and also because of how easy everything fit together. It started with Top #1 on his back and me sitting on his cock and leaning forward. Top #2 then came in from behind and slid his cock ontop of Top #1's didck and up my ass. Them Top #2 did all the fucking while Top #1 laid back and enjoyed the ride. I was still able to tighten my hole and they both came at the same time. Later Top #1 told be it felt as though his cock was being fucked which I thought sounded very very cool.
I've done it a few more times and have to say from my personal perspective it's ALOT easier if both tops are hung. I tried it once with two averagely hung men and it was really painful, though I thought it should have been much easier. They spent the vast majority of the time poking at my sphincter whereas the hung guys were able to move around and I got mostly shaft.
The best best best however is with one or both tops being uncut and DP'ing bareback....without a doubt the most incredibly easy and sensual DP ever. I know that's not PC but whattareyagonnado.....