It's happened a few times. It was fun.
If you're fooling around as a couple, you HAVE to be incredibly SECURE and trusting of each other. Otherwise, it would not work.
We hook up with guys using online profile pages, mostly. We like to make sure that anyone we're hooking up with is into both of us, otherwise if can create an obvious sense of favouritism.
In terms of the actual sexual acts, I find it really exciting. It's fine if you wanna step back for a bit and just watch. Or, the possibilities of who can fiddle which which bit of whom when are endless.
My hubby and i once spent the whole weekend with a couple who'd installed a sex dungeon in their basement. it was a whole weekend of hanging out naked, fooling around, some proper heavy sessions, etc.
It's not something we do every weekend, but a fun experience, if you find the right guys to do it with.