Well if you're working with 2 servers, there are two options I'd suggest.
1. Run a cron job every 5 minutes to rsync the htpasswd file for SiteA to SiteB and the htpasswd file for SiteB to SiteA. This works if you don't have huge member lists and its not particularly important for the person to have access to the second site immediately (they have to wait 5 minutes). The down side is they have enter their username and password again when they go to the second site.
2. Use
mod_auth_mysql and have each server connect to their own local database and the remote servers database. The downside to this is you have to have mysql running on both servers and listening on a public ip (unless you set up a vpn between the two servers). The user still has to enter their username and password a second time when they try to enter the second site but they can log in immediately.
3. Use
Apache::AuthCookie and use a token based authentication system for clicks between the two sites. This is by far the most complex system to implement but the most userfriendly. The downside to this is some software blocks cookies and makes it difficult for users to login.