There has to be some script that does it. It would check a referring URL and then if it was acceptable, would bypass the username/password requirement and allow access.

Some feed (plugin) links go through a cgi script, so maybe it checks the referring URL, and then allows access through a user/pass set up for that referring URL.

The other option, like Bec suggests would be to have your billing company set up something that writes the username/password to two separate password files, one for your site and one for your hubby's site.