How to display results from multiple indices?

It's actually pretty simple, and can be done both for the Autocomplete and the Instant Search results page.

1/ Multi-Category Autocomplete

You can follow our multi-categories autocomplete tutorial that explains how to perform the same query on multiple indices and display results in the same auto-completion search box.

2/ Multi-Category Instant Search results page

The easiest way is to duplicate your search code: doing a search on two indices is the equivalent of doing two separate search operations.

If you're not using the Javascript helper, but an API client directly, you can also batch your queries.

As a side note, searching on N indices will generate N search operations, which has an impact on the pricing.