How to use PowerShell to replace user domains

I have users who have 2 domains @domai1 and @domain2. Currently when a user logs into SharePoint with their @Domain2 credentials, they are not able to see the documents that they had access to when using their @domain1 credentials. I would like to use a PowerShell script to replace/merge all @domain1 users with the @domain2 equivalent. 

Has anyone ever done anything like this? How do I go about this?