I've done both once and IT WAS fantASStic!
First was couple who wanted a DP with hubby in her ass and me in her pussy (w/o a condom at their request). I was lying down as she climbed onto my cock in an all 4 stance and waited for hubby to get in her ass. Once he was in my job was to slowly start fucking her. THE MOST AMAZING SENSATION is that I COULD FEEL HUBBY's COCK just on the other side of her pussy. I mean that membrane is THIN and hubby was saying OMG's every time I stroked my cock into her pussy as I fucked her. It was so WONDERFUL to be their first and doing this in such a way that ALL THREE of US REALLY ENJOYED IT. It must be that hubby's cock in her ass AIMED MY COCK into her pussy in a totally new manner because her orgasm was shuttering! What a wonderful experience.
Second was a DP in the woman's pussy. In this instance, the hubby was lying on his back as his wife mounted his cock and placed a pillow underneath his butt so his cock was further inside her without her having to strain her back. Then I entered her pussy and stroked. ....I'm smiling right now as I type recalling the look of pleasure on hubby's face and he felt MY COCK glide over his back and forth, back and forth, back and forth. Once again, the wife enjoyed it too because our combined girth gave her own pussy more pleasure. And now I am smiling again as I recall the "renewed look" on hubby's face as he shot his load and we had more lube to share.
The latter couple (DVP) invited me back so hubby could watch me fuck his wife and then eat her pussy and we talked about me being the first cock in her ass butt we did not get around to that because fucking her pussy and licking it after fucking it became a "joint pleasure!" Life REALLY IS GREAT when people like us can share such wonderful lust with such patience, passion and courtesy.