This is a terrible idea: it involves exposing these unsecured clusters to the public internet. It's only a matter of time before someone deletes all your data, or worse.
Even with security enabled you should still not expose clusters to the internet. It's almost certainly a mistake to even have a public IP on the instances running Elasticsearch nodes.
Instead, you should set up VPC peering between your two regions so that the two clusters can see each other without also exposing them to the rest of the world.