The difficulty I have encountered is if both women are hetero or somewhat uncomfortable with another woman being there. In such a situation, I found myself having to spend more time making sure that each woman was feeling equally attended to and no one was feeling slighted or "left out". No one wants to feel like they are the "second string" in such a situation. It's the same in a 1F-2M threesome. If the 2x aren't comfortable around another x? It's going to be awkward for everyone. I've also been in the situation where both women were comfortable and at ease with the situation. They were both (in their own description) "mostly lesbian" but willing to tolerate/accept a man in the mix, provided he understood the rules of the game. I ended up rooming with them and learned a great deal from those women. Much of which was humbling and potentially ego-crushing, but in the end, I learned a lot of things that I otherwise would not have. They liked having a low-stress, no ego, open to discussion and sharing male, but were more than happy to lay out the situation if I every started acting (in their eyes) "too macho". One of their favorite ways of explaining that I was there at their invitation and always on probation? "You know, we could replace you with a wind up toy, a harness, and some properly sized/shaped silicone". Which taught me how to leave my ego at the door, relax, smile, laugh, have fun, be able and willing to listen as well as talk, and to stop worrying about "who's in charge" or any sort of similar silly ego-based role. We spent a lot of time talking and laughing. It's amazing how much better intimacy can be (regardless of who's involved) when there is the ability to ask, listen, learn and openly discuss what is on your mind instead of having to play silly games or look for hidden signals.