EU Tax Code - OffSet Tax

I need to setup an EU VAT Code for AP so when we receive an invoice in Portugal from a German Supplier we will apply a positivie line +21 and another negative -21, this is an Offset Tax

I know that I should setup both codes the negative as offset, but my question is where are those codes linked so when I select the one in the AP invoice both lines, one positive and one negative are created?

Thanks in advance.