pre_get_posts sorting on multiple meta keys

functions.php

add_action( 'pre_get_posts', 'sort_by_lname_then_fname' );
/**
 * Sort by last name then first name. `lname` and `fname` are the meta keys.
 *
 * @param  WP_Query $query   The query. Passed by reference.
 * @return void
 * @link   https://support.advancedcustomfields.com/forums/topic/pre_get_posts-order-posts-by-two-different-meta_keys-acf-select-field/
 */
function sort_by_lname_then_fname( &$query ) {
	if ( $query->is_main_query() && $query->is_post_type_archive( 'speaker' ) ) {
		$prefix = 'bu_conference_speaker_';
		$meta_query = array(
			'last_name' => array(
				'key'     => $prefix . 'lname',
				'compare' => 'EXISTS',
			),
			'first_name' => array(
				'key'     => $prefix . 'fname',
				'compare' => 'EXISTS',
			),
		);
		$query->set( 'meta_query', $meta_query );
		$query->set( 'orderby', array( 'last_name' => 'ASC', 'first_name' => 'ASC' ) );
	}
}