Linking accounts in this way would open products to being stolen. Fab already has enough problems in that department.
If you already own the products on an account, there would be no issues with downloading files and adding to a project in another account because you would have ownership over both accounts. Even if it’s not as convenient to switch accounts all the time.
You also have the option of being signed in on the Epic launcher on 1 account and in your browser for uploading on your other account.