Done this 3 times in my life under different circumstances.All three times were kind of disasters.Recommend having the girl find out a ahead of time if the dick sizes are similar or if one of the guys is bi curious and the other is not..That may sound messed up, but can only speak from experience.I'm a bigger guy downstairs and it mattered.
1.Older couple i met off AFF. I was mid 20's.They were early 40's.She was SMOKING HOT.The minute the trousers came off I could feel the hostility from him.The more she liked it the crabbier he got.Finally he just stormed into the living room.Guy basically told me to get to fuck out.Messed up part was we had driven to their cottage and my car was about 45 minutes away at their house.I played nice on the drive back(she didn't come) and the minute I got out of the car i grabbed him and ..well suggested he not contact me again.
2.A female friend(not even a friend really ..a fuck buddy off POF) wanted to have a mmf and had another guy.We get to the Motel( classy i know) and get down to business.This guys a fat dude, and i'm comfortable with my sexuality but this dude was jelly roll fat.He can't get it up.....and then says something to the effect that he was only doing it for her...he loved her.blah blah blah while I'm literally standing there with my cock in my hand.He and her go outside...and then i hear crying....yup..it's him....FUCK.I just left discreetly.
3.I met another couple off POF. My age.Early 30's.She is very fit..girl next door look.He was okay.Kind of mousy and weird.We end up at my place in my bedroom.He's kissing her and i'm going about some pussy play.She and i end up in a 69, while weirdly enough he's still clothed sitting on a chair at the end of the bed.We're enjoying each other about 3-4 minutes and i hear"God sweetie, your cock is yummy.."..followed by"it even looks yummy"(dudes voice) ANNNNNNNNDDDDD!!!!!! we're done.
I was very clear that i was straight and politely, but firmly, said i had no interest in his curiousities.i hooked up with her two weeks later and fucked her for like 2 months behind his back and didn't feel a inkling of guilt.
My point of these experiences is that i've found two women get comfy alot quicker then two dudes and certainly a lot more things can go wrong I've found.