So you have 2 different user tables and 2 different login systems on the same site?
the "easiest" way to correct this would be to edit one of the login pages, edit the authenticate user behavior and change the name of the user ID session that will be created
You will then need to create a new logged in rule that uses the new session variable, and edit any pages that reference the session directly to reference the new one.
However: This is really not a very good way to set the site up.
the best way to fix the problem would be to use only one user table in the site, and set up user level authentication to allow different access levels to different users in the system.