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 blue dollar sign beside the Charge Item (i.e., "Class Fee")

Edit the Discount name by clicking the pencil icon

Add the amount of the Discount and click Save.