http://docs.unity3d.com/ScriptReference/AssetDatabase.FindAssets.html

This page shows an example in the bottom of the page.

The comment in the code sample says:
// Find all Texture2Ds that have 'co' in their filename, that are labelled with 'concrete' and 'architecture' and are placed in 'MyAwesomeProps' folder

According to the documentation above, when using multiple label or types used in a search are ORed together.

The comment should say this then (notice the OR)

// Find all Texture2Ds that have 'co' in their filename, that are labelled with 'concrete' OR 'architecture' and are placed in 'MyAwesomeProps' folder