Is it possible to use search api indexs together?
Yes, you need to used the Search API multi-index searches module.


  This module is an addition to the Search API module which allows users
  to search on several indexes at once.
  
  When the module is enabled, you'll get a new Views base table where
  you can search all enabled Search API indexes at once. Performance
  might suffer, though, if not all indexes lie on the same Solr server
  (or other backend server that supports this feature).
  
  Please note that, since multi-index searches aren't supported
  explicitly by the Search API, most other Search API add-on
  functionality (facets, autocomplete, saved searches, search pages,
  etc.) won't work with searches created with this module.


There is also a sanbox project that allows you to combine Search Multi Index and Facets.