Hi,
Are you asking about connecting an AEM on-premise Assets instance with an AEMaaCS Assets instance? If so, there is no out-of-the-box solution because these are two different servers. You will need to come up with a custom solution to sync the assets between the servers. This solution will be dictated by several factors, such as network configuration, data volume, synchronization frequency, security requirements, and the specific use cases you need to support.
Hope this helps
Esteban Bustamante