Double Barrel Canon?
 

I'm thinking about getting a shoe mount ball head to mount my HV20 with wide adapter to the hotshoe of my XH A1. That way I always have a wide master to cut to, no matter what. Before I run out and try it, has anyone done this before? I'd love to hear about potential pitfalls with this kind of setup.

If you bump the camera and have to cut to the wide shot, the wide shot's going to have the same bump.

However, it is definitely something you should try, so you can report back just what pitfalls there are.

I have a similar but different set up as you suggest, but instead of attaching to the camcorder I attach my HV20 to a homemade bracket attached to my tripod, I use this set up to video weddings and being a one man operation it keeps carrying too much equipment to a minimum.
I use a relatively lightweight (but pretty steady) Velbon tripod with the bracket attached to the extendable centre column - this is good for filming the guests and their reactions etc in the congregation (for cut scenes) while the main cam stays on the couple.
It works well and I can pack it away fairly quickly to get outside after the vows etc have finished.
It obviously doesn't move with the main cam as Noah is proposing but it can be angled in any direction and left for a broader shot. I also use a lanc remote control to stop any unneccessary movement (knocks) of the main cam/tripod.