Connecting Neptune with OpenSearch

Have you seen the Neptune-to-OpenSearch integration? This uses Neptune Streams to keep an OS index up-to-date with a Neptune cluster. This is primarily used to enable Neptune's Full-Text-Search feature. It requires a separate OS cluster per Neptune cluster, as the index name that is used by this feature is hardcoded. So if both of your Neptune clusters have different datasets, you would need two different OS clusters, one for each Neptune cluster.

If the intent is to keep a bespoke OS index updated from data written to Neptune, you can modify the poller code in this stack to build whatever index to you want. You could even modify this to use two different named indices in OS to maintain just a single cluster, but that would only be for non-FTS use cases.

https://docs.aws.amazon.com/neptune/latest/userguide/full-text-search.html