In this case, the purchase makes sense if you need two of the same item. For example, two half-priced tubes of toothpaste will get used over time and may save you a little money as well. But, if all the costs are disclosed up front, you can make an informed decision. Con artists often try to pick up where responsible marketers leave off.
Con artists try to bury disclosures in the fine print or forget about disclosures completely.
Often these deals are too good to be true. Look at the offer closely. The catch with the vouchers is that they are usually extremely difficult or impossible to use. It's fun to get a good bargain. But use your common sense and avoid deals that are too good to be true.