Hello, it shouldn't happen. We respect domain of the website. So if you have credentials for domain bankone.com stored then we will never offer it form banktwo.com or anything else.
With subdomains we only offer it for domain service.bankone.com or similar. Could you please double check that you really don't have same credentials stored for both banks. Note that we may not be only one who is filling login form - browser can do it and/or another password manager extension. If you are convinced this is a bug, please contact me via kvetko@avast.com so we can resolve it together. Thx