Shopify does not allow customers to use more than one coupon or discount code per order. Most merchants suffer from this problem, and you can check on the Shopify forum how huge it is. Since 2019, it has had over 400 responses, the bulk of which are from merchants merely voting for or seeking this functionality. Shopify marked the topic as "solved" for some reason, although the truth is far from being solved.

A merchant commented there to ask for multiple discounts stacking saying:

"Multiple discounts needed here as well. Crazy this is not an option for businesses. Please implement."

One of the official Shopify representatives named Peter said:

"With any feature, we have to consider how it impacts other parts of the platform, along with any possible negative outcomes it could have towards you, the store owner. While I can't speak to whether this is an easy addition or not, I can assure you that each and every piece of feedback you've provided here in this thread has been added to the feature request. Beyond this, I am unable to provide any further insight, or context beyond your feedback has been shared internally with our product team for discussion/review."

The Shopify team certainly pays attention and listens to its consumers, but no official answer has been given as to whether or not this will be rectified in the future. Theoretically, it would need a significant amount of backend work on the platform. On Shopify, the entire discounting sector has always been a problem.