Originally Posted by Archbaker
This would be a weird thing to happen with Asturian influence on Franks or vice versa. In fact, Asturians and Franks should have very little reason to be at permanent war with each other as long as they havde more obvious enemies.
The same goes for most variations within Christianity, and possibly between pagans as well. There is no reason for happiness in a Roman Catholic area to decrease because it is next to a Celtic or Asturian one.
Based on that, I would advise against religions that are this similar. But maybe you already made that decision and I am way off on what your religions are.