Make 'First Last' default-style display name

gistfile1.php

/**
 * Build 'First Last' user display name
 *
 * Sets up default-style Display Name for users on new registrations
 * 
 * @param int $user_id
 * @uses wp_insert_user()
 */
function ww_default_display_name( $user_id ) {
       $first = get_user_meta( $user_id, 'first_name', true );
       $last = get_user_meta( $user_id, 'last_name', true );
	// Let's get together now
       $display = $first . " " . $last;
	// Update user
       wp_update_user( array( "ID" = $user_id, "display_name" = $display ) );
}

add_action( 'user_register', 'ww_default_display_name' );

/**
 * Filter display name before saving
 *
 * @param string $name
 * @uses sanitize_text_field()
 * @return string $output
 */
function ww_filter_default_display_name( $name ) {
	if ( isset( $_POST['display_name'] ) )
		return sanitize_text_field( $_POST['display_name'] );
		
	if ( isset( $_POST['first_name'] ) ) {
		$output = sanitize_text_field( $_POST['first_name'] );

	if ( isset( $_POST['last_name'] ) )
		$output .= ' '. sanitize_text_field( $_POST['last_name'] );
	}

	return $output;
}

add_filter( 'pre_user_display_name', 'ww_filter_default_display_name' );