Displaying two currencies on the bill/receipt

Croatia is in the process of adopting EURO as base currency so till the end of the year it is set by the law to show prices in both currencies, EURO and HRK (Croatia Kuna).
Is it possible to set that up?

On sales invoices only, you can show the total amount in your base currency if the invoice is in a foreign currency (because the customer is denominated in a foreign currency). Check the box:

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But this only covers the total, not prices or other values on the invoice. And it would require designating all customers in the Euro while you continue accounting in Kuna. So it is probably not a satisfactory solution. You may have to resort to a cumbersome use of custom fields.

Realistically, the easiest thing to do would probably be to create a parallel business with the Euro as base currency. Duplicate every single transaction for the rest of the year and provide duplicate invoices to customers, one in each currency.