Either make your own UG or use individual W8 form for US company (reverse charge in DE, no VAT). Still, you need to have at last 20% of your income from another source if you stay a freelancer. Ideally if the US company had two legal entities and you could issue invoices to both. Most people start their own GmbH with all the increased taxes it brings.
I don't know, it's best to discuss with a lawyer of course. Still, if they are two different entities with two different TINs, why would that matter to the tax office?
