I've had a couple of ffm from AW, all over ten years ago now.
I found them a bit of a performance. They know each other better than me and have their way of doing things. While they were all good experiences, I almost felt played.
As for a MF couple, I'm nervous that I might feel the same, sort of left out. I've had experience with a married couple where he was unable to perform, sometimes he watched and joined in, others he watched to next door, but that wasn't a paid gig, just an arrangement between adults, so I didn't have the same expectations.
I did have an mmf booked in March, but the crap weather stopped it :-(