Multi-sites, Same Users, Different Versions - Possible?????

Is this possible: 2 separate sites that share users but are different versions? I like drupal 6 forums, but many of modules I am using are 5.7 only. So... I want to theme a 5.7 site and a 6.1 site the same (so there is no difference), and use the 5.7 version for everything but forums. I want the 6.1 version to handle only forums, but not require visitors to login twice. possible?