How does proration work with multiple coupons?

When prorated credits and charges are created in mid-billing cycle subscription changes, their associated discounts will be prorated at the individual coupon redemption level for each line item. For example, if the line item had 3 different coupon redemptions applied, each of the 3 resulting discount portions would be prorated individually and then summed to create the line item discount amount for the prorated charge or credit.

To learn more about Coupon Proration: https://docs.recurly.com/docs/multiple-coupons-per-account#proration