The Missouri Tandem usually works best for threesomes of 2 men and one woman. One uses the vagina, whilst the other uses the anus. When the female is on the edge of orgasming, both the men suddenly pull out and flip her over, so the man who was penetrating her vagina is now penetrating her anus, and vice versa.
Bandit and I were giving Emily a good romp. When she got close, we gave her a Missouri Tandem! Good times.