Just something for OP to consider if he and his partner haven't tried an FFM/swinging before, make sure to spend some time at the beginning of the booking discussing boundaries with the WG and make sure your other half is comfortable with them.
During my first FFM with a punter and his partner I couldn't tell if she was nervous or reluctant but she said she was fine with everything. The first time he and I kissed she burst in to tears and left the room! She was doing it just because he wanted to and it meant she was miserable, he didn't enjoy it and I felt awkward as hell...
I would also second BustaN and let your Missus take the lead in selecting the WG to try and eliminate any insecurities she may have. Be prepared to take it slow, the more relaxed and at ease the ladies are, the better the booking with be. Especially if you want girl on girl!
xx