It's because of a few things in my opinion: the tightness caused by the pussy being stuffed full of cock(s), the actual sensation of glans on glans friction which is exquisite in that warm, wet environment, and the knowledge that as a guy you wont usually get off by rubbing your cock against another guy's cock. It's like a taboo thing in a way, and that thought that another guy's cock is giving you as much pleasure as this soft, wet pussy is conflicting, naughty but oh so good all at the same time. This was why I could not contain myself for more than a few minutes first time I did this.
The feeling of rubbing your cock head against another cock, inside a tight, warm, moist pussy is exquisite! The first time you do it you'll blow your load in a couple of minutes! Absolutely the best