How can I schedule an appointment with a couple?

You can do that by creating a group for that couple, naming it accordingly (so that you can recognize it, e.g.: "Smith couple"), and assigning the two people to that group (from CONTACTS > Contact Name > "Edit info" > "Groups Information" section > check the group name and save).

Then, when you schedule the appointment, you schedule it as an appointment "with a group" and select their group from the drop-down list in the scheduling pop-up window.