ReinieR- Together Unlike what other banks offer, all bunq accounts are full bank accounts, so they must be declared separately. There is no such thing as a secondary account with bunq .
Furthermore, people tax the assets in Belgium and that means that you have to declare everything, not just the balance of the 'main account'. That makes selective reporting possible and that is (also) not allowed in Belgium. You must declare the entire assets throughout the world. This also applies to VAT in the Netherlands.
You are therefore advised to include all bills in your tax return. If the declaration leaves room for this, then you could declare it as one capital, by adding the balances together and indicating that under one denominator, as long as all capital is included.