Use extra postcode input at checkout

I’m not sure this is the right forum? But I have added an extra postcode field to the checkout and it is used to have the customer add the postcode first, to see available shipping methods (as some are not available to some postcodes). It works, but needs some finetuning.

So I wonder if anyone else have done this and what can be done so that the input in the first postcode-field will be set into the second postcode-field as well (right now the input won’t go into the second field automatically).