The best way to make CMS A and CMS B to share their user account system is set one of them as the base system and yoy have to code, for the second CMS, sort of a API to make the communication. Like have a table ipb2chevereto where you tell the system that ibp uid 1 is Chevereto user id 3. Or you can also use all the IP login thing instead of the Chevereto system but like I said, that needs custom coding. Best way to doing this is replacing the login class and make your own with IPB to make Chevereto understand it.