Passing Multiple Relation Criteria In Plugin
I need someone more experienced with Craft to vet this, but it seems that this is working:

Supplying an array to relatedTo returns results if it matches one OR the other entry.

$projectCriterea = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$projectCriterea-&gt;section = 'projects';
$projectCriterea-&gt;relatedTo = [$themeEntry, $yearCat];
$projectResults = $projectCriterea-&gt;find();


If you supply "and" as the first entry in the array, it only returns results that are related to BOTH entries.

$projectCriterea = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$projectCriterea-&gt;section = 'projects';
$projectCriterea-&gt;relatedTo = ["and", $themeEntry, $yearCat];
$projectResults = $projectCriterea-&gt;find();


This was figured out by tinkering and guessing, hence I'm not sure it's the "correct" way to do this. It also doesn't let me search individual custom entries type fields, but this does do what I was after.