I'm not sure if 1password is intended to work alongside Apple Keychain, but I've been using them together for a while now -- using 1password to generate and save passwords, and Apple Keychain for the convenience of auto-filling login forms on website. They work OK together, but it exposes a conflict that could be considered a bug in the extension.
When Safari auto-fills a login form, the 1password extension always asks if I want to save the login, even if it's already saved.
I'm guessing the reason is that the extension simply asks itself, "Did I enter this login/pass?" If the answer is no, it blindly assumes it's new.
It seems like better logic would be for it to ask, "Is this login/pass in my database?"