Hi,

You need to spend some time on the design on this. There are decisions that needs to be made. Like should you push the data from one instance to the other or do you want to pull. Do you want to run it scheduled or should you have the attachments sync as soon as they are added etc.

Have a look at the solution of this old community post for some inspiration.
https://www.servicenow.com/community/developer-forum/copying-attachment-from-one-instance-to-another...

Regards,

Niklas