Function two_accounts_in_one 

pub fn two_accounts_in_one(bank: &Bank, account: &BankAccount) -> bool

Whether a card’s names cannot all be one account: two of them with balances on the same day that differ. One card then covers two real accounts: the newest balance alone is shown (balance_of), and movements the banks gave equal ids are read as one (review of 10 October 2026, R11). Said calmly on the card, for you to give each account a card of its own.