I use the same account details for both the US and UK amazon sites, and have a single item for both of them in my 1Password. In that item, I have both the URLs for the US and the UK amazon signin pages.
Whichever URL is first works fine. The URL that's second doesn't allow auto-filling.
In other words, I can auto fill the US site by changing the order of the URLs to make the US site first and the UK second. This breaks the UK site. If I list the UK URL before the US one, the UK site auto-fills, but the US site no longer does.