Well you've heard a ton of good advice already but I might add a few thoughts:
Our first MMF wasn't really that great either mainly because the guy, although sincerely wanting to please M, didn't listen real well when told what it would take. Not that we gave him instructions, but if a woman says things like "do that only softer" for instance I'm going to listen. So that was our first experience.
After that I was a bit like you in that I wanted to be certain everyone was on the same page and had a clear expectation of how we all saw it happening.
My first and foremost concern was that my wife would have a fantastic time. I did some "drilling", but mine was drilling into my wife I was OK with her and another man and the only way I would be dissapointed was if she spent too much time worrying about me instead of enjoying herself! I personally didn't have any insecurities to deal with. So I could tell her that and mean it. I realize that's not always the case for everyone. I also knew I wouldn't have any problem commuincating to either of them if I wanted in on the action or needed to take a break or whatever.
I made up my mind I was OK with with other guy naked and we might accidently touch. I made sure he was OK too. IMO if a guy is freaked out about that he might want to reconsider if he's really cut out for MMF.
You made a comment about what if they are going at it forever and you get left out. I would tell them both that was a concern you had and then if I wanted to "take a turn" at something when you were all together I'd let them know in a nice way.
Like others have said we feel it's all about the woman in a MMF. She should feel like a goddess or a slut whichever turns her on.
I like to do a little planning before hand but sometimes it can be fun not to plan at all. But our experience has been better with a bit of planning. Since you know this guy already, you can easily make sure you're all on the same page.
I emailed the guy with all the little tips that I knew would take her over the top from 10+ plus yrs of learning. M knew I was emailing this to him and ageed not to read it.
The end result was she had a hell of a time with two guys that had her pleasure in mind. facelick
-D