Mike, after rereading your thread title I realized I didn't answer your question about how accurately to align your 2 scopes.
As I understand it, if you were absolutely perfectly polar aligned then it shouldn't matter how much the two scopes are misaligned. In practice though it does.
The further you are from exact polar alignment and the further you are from guiding on the same point that is central to your image the greater effect it will have on your image, normally in some form of field rotation. Length of exposure has an effect as well on the outcome.
That's how I had it explained to me. Having said all that I can't say I've noticed many problems, but then my polar alignment process is very accurate (arcsec accuracy) and I stick to fairly short exposures, rarely longer than 10 min. I also use guiderings so its rare that I need to guide at a distance from the point I'm imaging.
Don't know if that's helped or not
