The 1x1 RPs I've had the most success with have been with the other player in a GM-type role. In that, they had the plot of the story and knew where it needed to go eventually. This isn't to say they didn't ask for input or change the story based on how our characters were interacting, but they did have a clear goal in mind. These are the RPs that the story actually finished, which is a super success in my mind.

Less successful, but still on the 'mostly successful' scale, are the RPs that my partner and I plot out together. Now, the problem I have is I am pretty ok with making subplots, diversions for the RP to go on that aren't part of the main story. I'm NOT good with overarching plots. So when I have a partner who is good with the big picture, but less so with the little things, we tend to work well together.

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I'm also flaky has heck so like. I don't really drop RPs as much as drop off the face of the earth for a while. (Gotta love crappy mental health.) So sometimes it might be a month before I reply. Which some people aren't cool with, and that's totally their choice, I don't get mad at them about that. I used to be one of those 'Reply 6 times a day' sort of people, but alas working full time and trying to have responsibilities means that doesn't happen.

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But I think the best chance of RPing well together is if your writing style mixes well together. If your writing style and tone clashes, it's going to be a LOT harder to write the story.