You know, we've been discussing just that very thing, Wayne. Zipper loves the outside (she was rescued from outside life), and Dagney only goes out occasionaly (she was only an inside kitty), so we may just have to keep them inside & outside for a longer amount of time. At night, everyone comes inside (mostly on & around the bed), and there seems to be little trouble. Maybe if we try what you're suggesting, they would forget the other exists.
BUT, we have a pet door that everyone uses! There are 8 cats here, plus the neighbor's cat comes in to "hang out" every few days, as well. (It's like a busy "furry bed & breakfast" around here.)
You're right though, once the hateful pattern has started, there's not much that can really be done to reverse it. I am still contemplating new homes for a few, including Zipper, but it takes so long to find the "right" home.
Thanks for the suggestions! I'll keep everyone posted!