I have had decent luck doing the following at spas where I am a regular:
1. go in in the evening.
2. offer to pay 1 house, 2 tips for 4hand, but given the understanding that if another customer comes in and they need one of my girls, she can go and I'll tip her for the time she completes.
3. Move any hanky panky to the start of the massage, instead of the end, when you know both girls are definitely still available.
In all reality most of the time, even if another customer comes, she makes him come back once we start anyway, but it seems to alleviate the girls big concern that one of their best customers might come in and walk.