Place both members areas on one domain, for example
members.domaina.com/paysitea
members.domaina.com/paysiteb
one htaccess file for members.domaina.com and link from there to all of your other members areas. And yes you are able to share the same htpasswd file even if they arent the same paysites/domains (but yet your surfer would need to re-fillin the usernam/password) if they are on the same server.
Hope that helps
