Two payments cannot be scheduled at the same time. You can schedule an additional payment once the first one has been processed (which usually takes up to 8 hours), or you can cancel the currently scheduled payment and reschedule it for a different date or amount.
Keep in mind that payments scheduled for today cannot be canceled. If you’d like to cancel a payment, you’ll have to do so before the scheduled payment date. If there is no “cancel this payment” option visible on the Payments tab of your Petal account, you won’t be able to cancel it.