I agree with Chicup. The best way to have an MFM is with the husband of a couple you already play with.
We've done this two ways:
1. When the other wife was out of town but knew and approved, and,
2. With the other wife watching.
Both situations were later reversed in the name of fairness and fun.
We've done FMFs in the same way. Never have we had a problem.
Mr. Alura