I loved up a lady once while her hubby watched. It was his idea because he thought it would be fun, but when play time was over she looked at me with a wink and told me to come back anytime....and he never offered again. ;-)
There is something special about being 8 inches deep in her and talking to her hubby at the same time. I would certainly like to repeat that experience if it ever worked out.
I can certainly see where it takes a special kind of relationship to pull it off regularly though.