you can only pull in data from a remote source, not indexing into a remote source. However this shouldnt be a problem, you just have to execute the reindex from the other cluster?
VPNs and routing could still make that accessible, but barring that, other options include having both clusters share a snapshot repository (probably S3, so it could be accessible to both), and snapshotting from one cluster, and restoring to the other.
In order to be a shared repository, both clusters would need to be able to read/write to the same shared filesystem. This is why I recommended S3, as it's the most secure option for external storage that works as a repository for Elasticsearch.
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