Here's a verification site for Keychain Access using Safari not to be able to store Internet Password if you have two or more basic authentication accounts of every directories on the same domain.
At first, click here to make sure that you can add Internet Password on Keychain Access.
id: first passwd: first
Then, click here to make sure that you can add Internet Password on Keychain Access again.
id: second passwd: second
Quit and re-launch Safari browser to clear cache of authentication information. (It's easy to open last session window from history menu on Safari.)
One last time, go to first directory and make sure not to be stored Internet Password on Keychain Access.