I have had husbands do this for me and the wife countless times. If they are really into it, they will lick my cock and balls and her clit and labia all at the same time... especially when I am deep deep, and she is spasming around my cock. It drives the whole fuck into hyperspace. It is rare for him to take my cum in his mouth because the women usually want to feel it, but he will usually lick it out of her, and clean me too -- if he is far enough into the whole experience to be under us in the first place.
Another position is her on her side, me upright fucking her, and him with his face in the fuck from either the front or back.
I have done this all a bunch of times, with another woman doing the licking as well.
About 5 years ago I took the under position with a couple with whom I was doing an mfm, 3some. He was quite large and fucked really well. She was gorgeous. Their genitals were both very beautiful. Licking both of them was a gas and she climaxed dozens of times. She was sucking me most of the time, and I felt her climaxes as she took her mouth off me to moan, and gripped me really hard with her hand as she spasmed. When he came I was holding his balls and felt the cum pumping at the base of his cock. The view of the whole thing was terrific. I certainly did not refuse licking them both clean, and he returned the favor when it was my turn to fuck and his to lick.