In the staff interface you can select, for example, "Diaries" in the Subject facet, then "Antiquarians" again in the Subject facet, and it will list what should be collections of, or including, diaries of antiquarians.
But in the public user interface, you can only select one value per facet. For example, you can select "Diaries" but then the Subject facet disappears.
Whether such intersections are representative of true holdings depends, of course, on the quality and consistency of subject classifications across repositories and decades of cataloguing.
I do not know whether this is a bug, or an enhancement, or if the simpler faceting in the PUI was a design choice. But it is something which we've modified at the Bodleian, and seems more suited to incorporating into the core code than publishing as a plug-in, if there is a agreement that it would be an improvement.
Another option would be to implement checkbox-style facets, where selecting two values would return the union of both sets of results. Solr is capable of this, but the changes to application code would be much more extensive, so maybe something to consider for 3.x.x?
Just a note that the ASpace PUI 2.1 version lacking this feature was certainly NOT a design choice (most of development was spent on implementing the Cherry Hill design, not on much of anything at all, unfortunately, related to search and faceting). It’s a bug, so fixing it with this much-needed feature request would be fantastic!!! You could use multiple facets in previous versions of the ASpace PUI, just like you can in the staff interface.
The last option would also be really great, since another requested feature is the ability to exclude facets from a result set (e.g. http://collections.si.edu/search/results.htm?q=blog&fq=-object_type:"Blog+posts", where I’ve searched for “blog”, but then excluded “blog posts” from the format types, thereby significantly shrinking the result set).
Appears to work as expected in the PUI. One can serially facet by multiple different subject terms which narrows the result set accordingly.
Works for me in the test instance.
Fix remains in 2.8.0 RC