I would try it as well under the right circumstances-- probably something either like a gang bang where we each wait our turn or one where each role is clearly defined--i.e. one guy fucks the pussy; the other guy fucks the face with the girl in between. Also, I'd have to be really comfortable with the other guy and know he's not a switch hitter.
One of my fantasies (and I've almost asked another hobbiest if he was willing) is a MFMF with lots of partner switching (i.e. guys swapping out with the girls and no gay stuff unless the girls go at each other while the guys re-charge
)