Require address line 2 in the shipping address.

require_shipping_address2_checkout_field.php

<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

add_filter('woocommerce_checkout_fields', 'require_shipping_address2_checkout_field' );

function require_shipping_address2_checkout_field( $fields ) {
	$fields['shipping']['shipping_address_2']['required'] = true;
	return $fields;
}