Can I display facet values from multiple different indices within a single menuSelect widget?

No.

Although the widget allows applying the select values to multiple indices for searching multiple indices purposes - see this code sandbox , retrieving/displaying different values from multiple indices is not supported at this time.

To display different facet values under the same faceting attribute from multiple indices in a single  selectMenu widget, you may consider combining those records into one index and retrieving the facet values from the index.