Having a twin brother myself I'm not going to rip on Ed and Nick for being so close. Although my brother and I have never been as co-dependent as they seem, I know how hard it is when you hit that point where you need to venture out yourself as a single entity. I imagine it would be harder for Nick and Ed both being guys, they've probably spent every minute together for their entire lives. It's easy to say they need to "break up" but it's easier said than done when you grow up associating yourself as a twosome.
If I had to take a guess though, Nick seems to be the leader and Ed the follower of the two and I say that's been their dynamic since they were kids.