Here's how to add multiple Discounts to an Individual Payment Plan (NOT a general Payment Plan, but one that is customized for a certain Enrollee):
On the Enrollee's Billing page, scroll down under "Tuition/Active Enrollments" to the Future Invoices and Payment Plans box.
Click on the Edit button.
Under Scheduled/Installment/Subscription Payments, click the dollar sign beside the Charge Item (i.e., Tuition)
Edit the Discount name by clicking the pencil icon
Add the amount of the Discount and click Save.