My wife prefers women and it was a long-time friend of ours and some chick. She didn't really want to but I was fucked up and it was already a 3some. I basically told him to get naked and get in on these two while I was banging them.
It was akward, I could feel, afterward, but I kept the mood light so no one thought of it.
That was the only time we've had another guy, but like I said, that's just biased. The 3somes with women have all been great for all 3 of us.
If you either just want maximum pleasure (for both, ie mff, too) or either of you have some type of dom fetish then give it a go. Hell, you already said there is tons of trust, nothing else really matters.
Sorry for the late reply, I was tearing it up on a game.