Those are both good ones! I love Frick & Frack stories, and I find the Nick/Kevin relationship interesting too. Just to be different, I'm going to add that I find the A-Rok relationship interesting. Brian and AJ are complete opposites in so many ways, from their personal style to their whole lifestyle, and yet, in a classic case of opposites attracting, they mesh so well.
This video sums up my A-Rok love:
http://www.youtube.com/watch?v=cFGXQFhk3GsI love their sense of humor and the way they can just bounce back and forth off each other forever. I also love that AJ is the only one who still calls Brian "Rok."
As far as exploring it in my stories... I don't write a lot of combo stories; they're mostly either group stories or stories about one of them with the other four in basically equal supporting roles. So I haven't really delved into AJ and Brian's relationship any more or less than I have the other guys. But I have explored that idea of them being so different, yet so similar in some ways in the AU I'm writing, Secrets of the Heart. It's funny because they live completely separate lives in the story and only have one scene together (which is in the next chapter I need to post), but they're connected by tragedy and live lives that, while on different ends of the spectrum, are actually very similar in a lot of ways. It worked well to use those two because even in an AU, I like to keep their personalities as true to life as possible, and Brian and AJ just created such an interesting dynamic.
Rose and I have also explored their differences in Song for the Undead. Brian starts out as this happy family man who loves his life, whereas AJ is a suicidal addict who hates society, and as the world dies, Brian loses everything, including his faith in God, and completely falls apart, whereas AJ finally finds his place in life and thrives in the post-apocalyptic world. They don't exactly switch places, but their outlooks on life definitely change and become more similar to each other.