When initially setting up card payments, you can only add one account. You can add more later on.
Top Tip: If you instead need to edit the account details, see this article: Changing the destination bank account for payments
To add another bank account, go to School > Payments > Set Up > Card Payments > Bank Accounts for Payouts.
From here, select the +Add button and complete the fields then click Add Bank Account for Payouts.