A unique property of 69 is simultaneous mutual oral sex.
This is so-called the butterfly position. The key here, is that the woman lies face up on a bed or couch, with her ass on the edge, and the man stands or kneels on the floor and penetrates her. Their torsos forms a right angle. The male, may lift the girl's hindquarters freely, and the girl may spread her legs wide apart, or wrap around the guy, or straighten up and prop on the guy's shoulders. This position is called butterfly presumedly because when the girl's legs are wide apart, and when the pumping is active, her legs simulate the movements of the wings of a butterfly.
Mathematically, there are 3 holes and 2 pegs. The 2 pegs can fill any two holes simultaneously. Both pegs jamming into any single hole, may it be mouth, vagina, or rectum, are also seen in pornography.