Oh, I love couples. They are like little treats on the tour. The appointments always run over, and most time lots of fun can be had.
The best advice I can give you with this is, if you have no experience with being with a woman, get some lesbian porn to get an idea of the dynamic. And bring lots of toys. I have two strap ons, various vibrators, butt plugs, metal and plastic, a male vibrator as well, cock rings, masks, tape, the list goes on, I have all kinds of stuff.
Also if the couple is so inclined you may want to incorporate a bit of role play into the situation.
Scenario: Girlfriend catches him being a naughty boy, and wants to make him pay. So, she ties him up to a chair and invites you over, and he is forced to watch the two of you go at it, something like that.
You can start the play off in the bathroom, nothing hotter for a guy than watching two girls soap each other up and down, touching parts and all.
Sex with women is different, so I would suggest a booking of an hour and a half at least. We can some times need a bit more time to warm up to the encounter.
But it can be loads of fun. Good luck.