if your friend is thick skinned then that has the potential for greatness, could be a fine line though! Still, years to prepare so it'll be awesome. I've always thought picking a best man would be pretty awkward because for the past eight or nine years I've had the same two best friends on the same level, so I wouldn't know what to do and I doubt that's an uncommon predicament!
It won't be so much insulting him as just listing the reasons why he's not ready to be a proper grown-up, and repeatedly telling his girlfriend "good luck" and "I'm so sorry". The list will involve things like when he stabbed himself in the leg with a steak knife, and when he spat in my mouth. Stuff like that.
We've had a bit of a bromance for the past 10 or so years and were always obviously going to be each other's best man, so it was actually pretty redundant when he actually asked me. But yeah, I guess in a lot of cases (perhaps even the majority?) there's not a clear cut favourite for the position. You could possibly do some sort of weird thing where you have two, which I think happened in at least one episode of Friends and seems perfectly plausible. If you don't care about tradition and the religious side of it and all that nonsense, it should be fine.
