Q:

Mixing results from different Lucene sources

We have multiple indexes using lucene and we do a parallel fetch of results and show them in different search result pages. We want to unify them as a single mixed set results sorted by relevance. 
Is there any standard method to mix results of different indexes whose relevance scores are specific to those indexes?

A:

If the configuration for the indexes are similar you can merge you indexes into a single one.
Else, Solr Distributed Search is used to query multiple instances of indexes with the same configuration across machines.

When an index becomes too large to fit on a single system, or when a
  single query takes too long to execute, an index can be split into
  multiple shards, and Solr can query and merge results across those
  shards.

However, we used it to query across multiple indexes on the same machine to mix the results.
You would need to sync your configuration, however to be able to query the indexes.
Also, it has its own limitations you would need to check.

