Answer
It depends on the retailer. Many checkouts accept only one promotional code per order, while some retailers allow selected codes or automatic promotions to be combined.
The checkout result and retailer terms are authoritative. If adding a second code removes the first discount, compare the totals and use the better eligible offer.
What to check before checkout
Read the retailer’s current promotion terms, confirm that the code applies to the items in your basket, and make sure the expected saving appears before you pay.