Shit, just realized he said different servers :D
You could still use the multiple htpasswd module, but you'll need to scp your password files between servers every 5 minutes or something.. that's how we were doing our videos, which were stored on a seperate server, but allowed to be access from all our sites. Cookies or referrer were both buggy for that application, so we just synced up the password files and let the video area read from them all.
I just did mod_auth2.c though mod_auth20.c and packed them up in a tgz file if anyone needs them...
http://www.manicproductions.com/authmodules.tgz
It's all gnu, so it's all good .. I've only built them as DSOs, but they probably work fine compiled into the core too.
CGI