Short answer: maybe.

Longer answer: this varies feature to feature.  Typically you will find the answer in our "Shipping & other details" section, on the right side of our home page, down toward the middle.

If you see the language "only one [voucher] may be used per transaction on the partner site," well, you'll know that vouchers cannot be combined.  

FYI, the reason might be related to the merchant's shopping cart.  Many will not allow for the entry of multiple codes; a merchant would have to override this manually. Given how many vouchers they may sell today, we can't require them to do a manual override for all buyers.