How to make user id/permissions on external SSD boot match old OS on internal drive?

I want to boot OCLP Sequoia on a 2012 iMac from an external SSD but keep my existing Catalina internal drive as-is and possibly alternate booting them. Is there a way to make the login account id's match to have permission to access files on the opposite disk and any time machine or other external drives attached? I don't want to use migration assistant to create the account unless there is a way to keep it from copying. all the old stuff.