But in that case, I've noticed people specifically mention that they want a partner who can create and use other characters for that specific purpose, while the whole "doubling" thing seems to have a more specific function. A limited application, if you will. Plus, "double" is just another word for two. Not for "I want you to be able to play more than one character."
Though I was involved in a 1x1 on another site (before I moved and lost my home internet) where the lady and I started with two, then started another two, while throwing in various background/NPC characters. So in a way, we evolved our RP into a doubling, going from two to four "main" characters. Heh. When I left, however, we were on our way to throwing in a few more "main" characters. Heading for a rather large "cast" we could throw together once we reached a point in the storyline where we intended to have a serious "situation" develop (the story was taking place on a "cruise" starship).
So yes, filling roles is not also necessary for story flexibility, but it's also quite fun. Can lead to very interesting turns. Even in a "romance" RP.