My wife is an identical twin. She and her sister share all that telepathic empathy you read about. One is right-handed, the other left. When one broke a leg riding, the other, miles away, felt the pain. They text each other at the same time with the same message; cook the same food; send each other the same cards, buy the same bath cleaner...well, you get the picture.
On one occasion we all went off for a jolly weekend in Marlborough (it can be done, really). One of the twins spotted a pair of trousers in a shop window, went in and asked to try them on. A few minutes after she'd left the other spotted the trousers, went in and asked to try them on. The assistant gave her a very odd look. Later they both returned to buy them (at the same moment, naturally) and all was explained.
There are upsides. At school, they were never short of a friend. Fall out with them, and it was two against one. When their own children were small, each had an instant, almost indistinguishable substitute mother for baby sitting.
There are downsides, too. All those strangers who come up and greet the wrong twin like a long lost friend, or take umbrage when they think they have been cut dead. The impossibility of either husband ever separating the twins by moving away - you marry one, you take on both. Metaphorically speaking, that is; they never secretly swapped boyfriends. I don't think.
And here's a funny thing. Not only do the twins share a birthday (duh), but their husbands (who are not remotely related) share one too. What are the odds of that?