Some background first: Our company "XS" is a small spin-off of a larger company "XL". Our mother company "XL" has a MS Dynamics CRM setup. However, for several reasons (technical and legal) we need to run our own Dynamics CRM at "XS".
Currently "XS" users login via "XL" SSO to Teams, Dynamics, Sharepoint and other MS tools using their email accounts of "XS". The important thing here is that users may rely on accounts and data stored at "XL" for a longer period. They need to be able to access both corporate accounts / MS tools in parallel, while the CRM needs to be migrated into an own account.
How would you migrate from the CRM at "XL" to the CRM at "XS", while allowing users to access both XL and XS accounts in parallel?