Schools usually try to put twins in different classes. In part, it's for the convenience of the teacher - identical twins can be hard to tell apart. But the main rationale is that if you separate twins, they will make more new friends. Isn't that great?
If you've had a week of intro economics, I hope your answer is "No." Consider: What would you do if aliens abducted all of your friends? In all likelihood, you'd wind up making a bunch of new friends, right? But it would be absurd to claim that the aliens had done you a favor. Before the body snatchers came along, you'd didn't need those new friends, because you were happy with the ones you already had. To argue otherwise is just make-work bias.
The same logic applies to splitting up twins. Sure, if you separate twins, they'll make more friends. But that hardly means you're doing them a favor. The reason why twins put less effort into making new friends is that they've already got a better friend than most of us will ever have. For twins, the marginal benefit of trying to making new friends unusually small - and cliquishness is their optimal response.
You could argue, admittedly, that kids underestimate the benefit of making new friends. But what reason is there is to believe that this is true? And in any case, that's no reason to specifically target twins. If you really believed that kids had an "anti-new-friends bias", you'd ask all parents to name their kids' best friends, and make a effort to separate as many children as possible from the kids they like the most. "You'll thank me later," right?
A decade or two ago, pundits often said that the Japanese economy was doing well because all their factories were destroyed during World War II. As a result, the story went, the Japanese built shiny new factories from scratch, and the victors with their musty old plants just couldn't compete. The standard economists' response to this nonsense: "If that's true, all we need to do to catch up to the Japanese is just bomb our own factories!"
Folks who want to separate twins are making the same mistake. We always have the option of "destroying in order to rebuild." But it's usually a bad idea - especially if you're destroying over the objection of the people you're trying to help.
So what should we do with twins? Simple: Ask them what they want to do. If they want to stay together, keep them together. If they want to separate and develop their individuality, split them up. And if you're really sure that "Stay together" is the wrong answer, why not try to convince us with a little self-experimentation? Cut the time you spend with your best friend in half, and tell us how you like it...