Merge results from multiple sources into one final results set for Autocomplete and/or InstantSearch

I’ve already found how to query against multiple indices, and managed to successfully do so, however the results are still segmented out to some degree.

Is there any way to merge the results from each and have one final result set that is ranked based on the final relevance and everything determined from the indexes?

Or if I’m wanting that, it’s best to just use the one index? One of the usecases is going to be multiple sites in a WordPress multisite network, and we’re seeing how to potentially have one search input search from all sites.