After I'd been here twelve years here with Adolf Mader, he and I actually wrote a joint paper. It certainly seemed to prove the adage that a joint paper requires twice as much work from each of the authors as if they'd written it individually.
The original idea was Adolf's, and I must say it looked like a pretty dim starter to me at first. But then we found a lovely idea that enabled us to do very simply everything that anyone else had ever done on the topic plus a bunch of stuff no one else had ever thought of. Strangely enough, the idea itself was not really new: it's just that nobody else had ever realized how useful it can be. So we wound up with a pretty decent paper.
I wish I could say that the final paper has a simplicity reflecting that of its core idea, but it does suffer from an abundance of results -- our little idea just had too many consequences.