It should be possible to use two separate login forms and tables and just set different session variable names so that you can have separate rules.
I'd have to look at how you have it set up to see what is wrong in your specific scenario. So... when you log into the back office and try to view a back office page it redirects back to the the public secured page?
I'd have to look at the login page, the page in the back office you can't view, and potentially the rules definition. It might be easiest to give me FTP access and steps to reproduce so I can access and update the files to debug.