DD (14) is best friends with another girl the same age, and I mean BFF forever and ever. It's like they're joined at the hip, and have been for 5 years.
Now although I'm not that fond of the girl - she can be pretty immature - generally i let them be, because at this age you can no longer choose their friends. We encourage other friends, but as my DH says we can't prevent them hanging out together.
The problem is it's starting to affect DDs confidence in that she isn't happy to do anything without the BFF. And vice versa, they rely on each other so much.
Now everything that DD does, the friend copies. DD joined Guides - so BFF followed her. DD joined drama club - immediately the BFF did too! For our own reasons we had to quit the drama.... and so did BFF. We suggested that the friend try out for some kind of sport, which she is good at and DD isn't, but of course that didn't happen.
Now DD has shown an interest in swimming, and we looked into suitable clubs. Thought it would be good for her to try something new and make new friends (bear in mind they spend most of their free time together, 1 hour once a week meeting new people isn't much is it?) DD came out of first swim meet, immediately phoned the BFF to tell her, and she is now going to find out about joining.
DH says there's nothing we can do, and any attempt to split them up would 'drive it underground' ?! 
I think if we do nothing we'll end up with a Heavenly Creatures scenario and someone getting murdered.
Sorry it's so long. They are both only children btw, which maybe explains their obsessive closeness.
*ages, sex and activities may have been changed to protect anonymity