Ditzy/Derpy being the slighty over the top character and Rainbow Dash being the more down to earth one that has to deal with her antics and make the best of it. I just like that combination somehow.

Hope this helps.

I answered this question once before, I'm not sure under which drawing it was though so, basically, ever since Ditzy Doo was mentioned and Rainbow Dash got annoyed I kinda thought they would make a great duo together. Then "The Last Roundup" happened and it set things in stone. Those two, for me, have a really nice Ying and Yang effect, a nice balance so to speak.