We have done just what you are talking about. Once while installing air conditioning at the house of a couple we used to play with, we fell into a MFM threesome. Of course them Mrs. Couple wanted hers..
We mixed it up a couple of time with either MFM or FMF threesomes with this couple mainly due to the trust we had built over time. We still all met and played together but we believe with trust comes even more fun.