It is very common in men to have the fantasy of having sex with two women at once, so if you are an escort you need to be prepared to offer this service. In general they ask you to bring a “friend” over, but there other times you have to deal with an actual couple.
First of all, you need to talk about what they are expecting with the encounter. The ideal thing to do is to keep the lesbian action the lowest as possible since the client wants to be touched and caressed by four hand. But, of course, there are some men who prefer watching two women having sex.
You also need to be aware that the encounter has to be fluid and organized. The best is to let the client take the movement and rhythm control of the sexual act. But there are times, when the man wants to be sexed up together with his wife. An important point to have in mind in this situations is to team up with her.
Ask her what her partner likes doing in bed and let her guide you. In this way, she won´t feel rejected at all. It is important too not to be romantic with the man, just dirty and casual talk. And while the couple is doing it, cheer them up, but don´t over do it.