Assumption: 2 person household.
30,000 euros taxable income
1. each person claim 15,000 euros as income no matter who earned what and each pay tax (14%) on the amount in excess of 9700 euros.
2. the 9700 euros is deducted from the 30000 euros only once and 14% is payable on the balance.
Either or answer will suffice 1 or 2. Do not complicate this further.