Johannes 🇦🇺 If there is not enough money on your billing account bunq will take the money from any other account you (co)own that has enough money. I don't know for sure in which order they try the other accounts. You can read this in Chapter 7 of the Term&Conditions.
If a direct debit comes in and you don't have enough money the direct debit is not made. You'll get a notification and you have to manually do something about it. It can either be transferring money to the account and paying the direct debit or changing the account for the direct debit. So it's just like when you get the first direct debit of a company.