If you have a gift card for a subscription (via your account), then it's not possible to use both codes at the same time. You can only have one active promo code on your account. Once the credits of one code are used, you can activate the other code.  

If you have a gift card for a single purchase and a promo code, it's possible to pay with both at the same time. Add the promo code first and then your gift card code. The credits of both codes will be deducted from your order total.

If you are unsure which gift card you have, go to https://shop.bloomon.co.uk/pages/giftcard-redemption where you can see all the different options and check which gift card you have.