Unfortunately, the WP query engine doesn’t provide support for more than one meta value in a query string. Multiple ‘meta_key’ or ‘meta_values’ are ignored (‘meta_value__and’ is also ignored, it’s not a valid keyword).
One workaround is to use a single value or a taxonomy instead. Or, a ‘save_post’ action can be used to set a single custom field value or category when the post is saved.
Another option is to create the map using PHP. If the query is passed as an array, WP allows complex mtea queries. You’ll need to add some code to your template:
Thanks very much for the quick response, Chris. I’m going to change one of the custom fields to a taxonomy. That’s probably a better approach for my site anyway, as the values for one of the two custom fields could apply beyond the particular custom post type I’m building the query for.