Hi @MartinLoeper,
Mirroring data set between MongoDB atlas clusters can potentially be done via the mongomirror tool, however, since it is a standalone process you need to reliably monitor and resume it at your own risk. This tool was designed for live migration purposes therefore was never tested for long duration tests which means we cannot guarantee if it will be sustainable or perffomant solution.
https://docs.atlas.mongodb.com/reference/mongomirror/
Alternatively, you can setup Atlas triggers linking both clusters to migrate subsets of collections from source to target.
Best
Pavel