I've done it a few times with a couple I used to 3some with. The way we did it was one of the tops, either myself or the guy's bf, would have him cowgirl style slide onto his cock and the other top then would slide into his ass as well from behind and fuck him. It's not something really that was too comfortable or really enjoyable for me. I think it's all for the bottom's pleasure. Only one guy can really fuck at a time. If both tops try to start fucking one cock usually slips out. It's best for one of the tops to stay in deep. I'm glad I experienced it though.