Well, I've been in a similar situation (though final project, not PhD). In my case, I was the "obsessive" one. I was really interested and wanted to know everything, but I was stuck with a person who just didn't bother. So after a while I simply gave up and did some of her work as well (because I had to wait for her to finish before I could continue).
So in my experience, making two students work too close, just isn't a good idea. I like cooperation, but this kind of codependence... It just slows one person down and stresses the other one.