Hi @Oliverhive ,
Thank you for posting in this community.
Currently, there is no direct method to sync files between two SharePoint sites that are on different Microsoft tenants. Regarding your concern, to improve the Microsoft sync view and this type of change has come up. As we listen to all feedback and consider them for additional feature/future versions of our products, and we requested you send/provide feedback our Microsoft feedback portal. Your feedback helps us know which features are most important to you.
For ongoing synchronization, you might need to consider alternative solutions, such as using third-party tools designed for SharePoint migrations and synchronization, or manually copying the files as needed.
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