I have been in a pussy while another guy enjoyed her ass. Tight - and feeling his cock move back and forth (through the membrane between anus and vagina) against mine as he stroked was a different sensation, and stimulating.
But that's nothing like the sensations of simultaneously sharing a vagina with another penis (double vagina penetration (DVP), or double pussy penetration (DPP)). I have had that privilege at least half a dozen times, with 3 different women. I was always with the two guys facing each other, and the woman between us, so the more sensitive undersides of our cocks were rubbing together. Usually I was under her in an upside-down missionary, or cowgirl-leaning-forward, position. The other guy entered her doggy.
I definitely felt the other guy's cock, especially the head, firmly sliding the full length of mine. It was very tight and I had no control over the pressure. The ejaculations happened pretty quick. Both guys try to hold off, because it's incredible to feel the other cock twitching and pulsating against yours as he releases his sperm. You'll probably feel the warmth, then the awesome feeling of instant "silky seconds" as his seed spills into her vagina and around your cock. When the other guy climaxed first, his orgasm almost always triggered mine.
Obviously, this is a VERY "bi" experience and the two guys need to be agreeable to it. (It always surprised me that who would meet a second guy for a 3-sum, and called themselves "very open-minded", were usually pretty homophobic and wouldn't even consider DVP, or the 69/doggy position.) The girl must be well-prepared - VERY open, relaxed, and lubricated, probably several preliminary orgasms. She will probably experience a bit of pain as the head of the second cock stretches her opening and slides in - similar to being taken in the ass, or the discomfort of being stretched open the very first time a penis entered her. The second guy entering must be very hard to get in. He is also in a position to possibly injure or damage either the woman, or the other guy, so I think a bare cock is definitely called for - he needs the feedback to tell if he's being too rough.
I think every woman I've ever heard talk about double vaginal penetration described it with the same phrase: "stretched and filled". That sounds kind of painful to me, but they seemed to enjoy it. I don't think the woman ever climaxed while both cocks were inside her, but feeling the second guy pull out tended to trigger her orgasm.