functions.php

<?php

/**
 * Order results by multiple meta keys
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/order-results-multiple-meta_keys/
 */

/**
 * Listing Query 
 *
 * @param object $query, WordPress Query
 * @return null
 * 
 * @author Bill Erickson
 */
function be_listing_query( $query ) {
	if( $query->is_main_query() && !is_admin() && ( is_post_type_archive( 'listings' ) || is_tax( array( 'listing-type', 'listing-location', 'listing-price' ) ) ) ) {
		$sort = isset( $_GET['listing-sort'] ) ? esc_attr( $_GET['listing-sort'] ) : 'recent';
		
		// Sort by Featured, then Most Recent
		if( 'recent' == $sort ) {
			$query->set( 'orderby', array( 'meta_value_num' => 'DESC', 'date' => 'DESC' ) );
			$query->set( 'meta_key', 'be_listing_featured' );
		}
		
		// Sort by Featured, then Price (orderby filter toggles low/high)
		if( in_array( $sort, array( 'price_low', 'price_high' ) ) ) {
			$query->set( 'orderby', 'meta_value_num' );
			$query->set( 'order', 'DESC' );
			$query->set( 'meta_key', 'be_listing_featured' );
			$query->set( 'meta_query', array(
				'relation' => 'AND', 
					array( 'key' => 'be_listing_featured',     'compare' => 'EXISTS' ), 
					array( 'key' => 'be_listing_asking_price', 'compare' => 'EXISTS' ) 
				) 
			);

		}
	}
}
add_action( 'pre_get_posts', 'be_listing_query' );

/**
 * Listing Orderby
 *
 * @param string $orderby, SQL orderby parameter
 * @return string $orderby
 * 
 * @author Bill Erickson
 */
function be_listing_orderby( $orderby ) {
	if( is_post_type_archive( 'listings' ) || is_tax( array( 'listing-type', 'listing-location', 'listing-price' ) ) ) {
		$sort = isset( $_GET['listing-sort'] ) ? esc_attr( $_GET['listing-sort'] ) : false;
		if( 'price_low' == $sort )
			$orderby = 'mt1.meta_value+0 DESC, mt2.meta_value+0 ASC';
		if( 'price_high' == $sort )
			$orderby = 'mt1.meta_value+0 DESC, mt2.meta_value+0 DESC';
	}
	return $orderby;
}
add_filter( 'posts_orderby', 'be_listing_orderby' );