Joint best man speeches are becoming more and more common. In fact, grooms are getting so carried away these days that the triple best man speech is making an appearance, and my record was at a wedding in the Middle East which involved 15 best men. This trend seems to have started because either the groom just can't decide between two people, or they're so wrapped up in themselves they think that only two men can properly cover the person they've become.
Whilst I'm not against having joint best men, it does present a lot of problems. Firstly, the combined length of the speeches. If you have two people making separate speeches, you could easily be looking at 20 minutes in total, however, it's usually way more than that. I've heard of a wedding where the two best men spoke for 45 minutes each, and that is a catastrophe of enormous proportions. I've got some great friends, but not in a million years would I like to sit through 90 minutes of speech hearing all about them, so imagine if you didn't know them at all? Torture, that almost certainly has some grounds for a legal response. Also if you have sequential speeches, you're giving one guy the dreaded last man spot. If you had 4 speeches before him, guests may have spent well over an hour sitting there listening to all of this, and all they want to do is get to the bar. Plus, if the first best man has really nailed it, you're on a hiding to nothing.
The best thing to do if you're having two best men speaking is to make it a joint enterprise, so they're up there together sharing the speech. However, I've written for a lot of joint best men speeches and invariably it consists of one motivated and enthusiastic guy, and another who's quite happy to wing it. The disparity in their approaches always leads to a lot of tension and resentment, and a bungled delivery on the day.
This could all be avoided. All a groom needs to do is give one best man the speech, and the other best man the stag weekend/bachelor party. Remember, if you as a groom are sitting there thinking about what you want out of your best men speeches, then you're missing the point. You have to keep the sensibilities and patience threshold of of the guests firmly in your mind. You already know all about you; this is for them.