1x1s are definitely a completely different animal. When you have just two people cooperating on a story, it allows for NPCs to be in flux. I had a roleplay which lasted well over two years, with over 50 characters between the two of us. The secret of our success was that either of us could introduce content, and either of us could alter content, but we both had to be okay with the changes. Most times, the person introducing new content was the one who had primary control over it, but that too was flexible.

When you get it right, you really can go the distance. That "give and take" you mention is necessary in any roleplay. I just think it's a little easier to deal with NPCs and how they should be handled when you're in a 1x1, versus a group play.