This API was introduced in release 7.0.
The secondary index search API is identical to the regular Search API, but it searches the secondary Solr index configurable through Search2.ini. API requests are structured the same as primary searches, but the “search” part is replaced with “index2/search” – e.g. http://localhost/vufind/api/v1/index2/search?lookfor=sibelius&type=Author