Sorting by Last & First name

functions.php

add_filter( 'um_members_directory_sort_fields', 'my_custom_sorting', 10, 1 );
function my_custom_sorting( $sort_fields ) {
	$sort_fields['last_first_name'] = __( 'Last & First name', 'ultimate-member' );
	return $sort_fields;
}

add_filter( 'um_modify_sortby_parameter', 'my_custom_sorting_handler', 10, 2 );
function my_custom_sorting_handler( $query_args, $sortby ) {
	if ( $sortby != 'last_first_name' ) {
		return $query_args;
	}
	
	$query_args['meta_query'] = array_merge( $query_args['meta_query'], array( 'last_name_c' => array(
		'key'       => 'last_name',
		'compare'   => 'EXISTS',
	), 'first_name_c' => array(
		'key'       => 'first_name',
		'compare'   => 'EXISTS',
	), ) );

	$query_args['orderby'] = array( 'last_name_c' => 'ASC', 'first_name_c' => 'ASC' );
	unset( $query_args['order'] );
	
	return $query_args;
}