Is it possible if two domains are on the same hosting unit and to sync (not threads since they are in different locations) but the users and their data?
Yeah. What you're looking for is called single sign on (SSO) by the sounds of it. Search for SSO by Waindigo and see if that does your job, it might but I've never really used it. If it doesn't, as I kinda expect, you'll probably need custom tailored development.
