You have to have a lot of energy to do two women at the same time and you can only really come once, so it's probably going to disappointing for at least one of them. As for having another man involved, I'd be worried about his intentions, i.e. is he interested in just the girl or do I have to watch my back. In any case, it's one thing to fantasize about situations but I don't think you can separate sex and emotion so easily in real life. It would be too easy for jealousy to become an issue or for someone to do something that totally turns you off.