Locate the appointment on the appointment schedule and click it.
To take separate payments from individuals, use the dropdown menus to the right of each persons name.
To take one payment from one person, click 'Pay all' at the bottom. This will pull up the sale for the whole appointment and will assign it to the personal originally assigned to the appointment. You can always change this persons name to the other individual on the appointment if needed.
Choose payment method and you're all set.