From previous experience, I think you'll find that most girls who offer MMF will be much more receptive if they're familiar with both of the guys.
If there's a WG you've seen regularly, I'd suggest bringing up the subject with her. She may well be up for it and also have other clients in mind who would be up for it too.
A lot of WG's won't openly advertise MMF but will often be receptive to it. After all, it's extra money for them in the same space of time as a regular booking.
