405735 Shared .htpasswd file

We have two domains with which we wish to share one .htpasswd file. As an example: Domain #1 - is a private membership and has its own protected membership area. Domain #2 - is a separate private membership and also has its own protected membership area. We want to allow all members of Domain #1 to have access to the private membership area on Domain #2. As new members join Domain #1 we want them to automatically get access to Domain #2 members' area too. Members of Domain #2 however would NOT get access to Domain #1's members' area. In simple terms - when someone tries to access domain #2, we want the system to look at the Domain #2 .htpasswd file. If the user is not there then it should look at the Domain #1 .htpasswd file and allow the user access if they are in that file - otherwise prevent access to Domain #2 If this is indeed possible, it should be a relatively simple task for anyone confident in modifying .htaccess files.