Checkout and portal V2

Customers can apply multiple coupons from their Customer Portal account in the earlier version of the portal(V2). To enable this option, navigate to Settings > Configure Chargebee > Checkout & Self-Serve Portal > Customer Portal > Allowed actions and enable the "Allow multiple coupons" option. 

Here's a screenshot.

Note that only two coupons can be applied to a subscription, with each coupon having different Discount Types(Fixed Amount & Percentage). If you apply a coupon with the same discount type, then the previous coupon will be overwritten.

Checkout V3:

Currently, your customers can apply only one coupon for a subscription from the latest portal account, however, we are working on supporting in the upcoming releases.

Alternatively, you can apply multiple coupons for a subscription from your end. In the subscription details page, use the Coupon button on the right to add the coupons to the subscription.

Via the API, you can pass multiple coupons as an array while creating or updating the subscription.

Related articles and documentation:

Coupons in Chargebee