Only one promotional code can be used per order. This is mentioned in the terms and conditions for each campaign. We are only permitted to make exceptions for gift vouchers.
For further details, please see:
How can I combine two gift vouchers to pay for my order?
Can I combine a promotional code and gift voucher in the same order?
We always advise you to make use of the promotional code that will expire the earliest. Keeping in mind, you can also place two separate orders with differing codes should that be more beneficial for your order.