Hi John, 

You can do this with a t-if  https://www.odoo.com/documentation/10.0/reference/qweb.html

Something like this 

<t t-if="o.partner_id.country_id.code == 'DK'">

                <span t-field="o.company_id.vat"/>

            </t>

Thank you Lara. It was of great help

If you have 2 different TAX numbers it means you have 2 different legal entities for which you might want to maintain 2 different chart of accounts i.e sets of accounting statutory reports. It is thus advised that you configure a multicompany environment in your Odoo installation.

This way you will be able to maintain 2 companies, say which employee has access to which company and most importantly print your business documents (eg. Invoices) with the appropriate letterhead and footer.

Here is an introductory link to multicompany settings: https://www.quora.com/How-do-I-manage-the-accounting-in-multi-company-environment-in-Odoo-formally-OpenERP-8-0

Hope it helps