if the both db's are on the same physical MySQL server then you should be able to access both db's, assumng that your ISP allows this through their security settings. thgis woulkd enable you to have one db with security settings in. however that begs the question why no have just one db for both sites
as an alternative, depending on what front end you are using, you could replicate the user account information in both db's by writing to both fromt he same script.. but thats klunky and not a smart idea