Wow, I guess I'm really lucky. We are pretty evenly matched, though we do use different-bike handicapping.

I ride my cross bike, he rides SS on longer, hilly rides and we're pretty evenly matched, though he was consistently pulling ahead on descents until I got new rims. They were totally worth it to pass him .

We also take turns in front, so there are no excess drafting accusations. Over the course of several rides, it all seems to average out. I really appreciate DH's patience, though admittedly, I think of it as training and he thinks of it as riding. (He's non-competitive.) So attitude is as much a factor as fitness (or sweet new rims!) in happy co-riding.