Hi, Micheal. Seems most of the replies thus far have recommended additional hardware etc. So I'll just mention the "best practice" for your current situation traveling sans laptop etc.
IMO the highest risk of losing electronic images is not to technical failure of cards, etc. The real risk is lost/stolen equipment or accidental format of a card prior to copying the images from it. Reducing that type of risks can be as cheap/simple as you like or can involve as much additional equipment/complexity as you choose.
The fundamental and simple step to reduce risk from the reasons above is to keep (at least) two copies of you images in separate locations. You mentioned that you already write to both cards while shooting so this can be quite easily done. If you're a high volume shooter it is really easy because both cards will fill at the same time and you then simply remove them from the camera and put them in separate locations. It can be a little more complex and require some disciplined organizational practice if you shoot an entire holiday on one set of cards. For example, if you start each day shooting by creating a new folder(in-camera) the you can use multiple cards in the secondary card slot and change the secondary card each day or after each important shoot. With two or three cards to use as secondary you can rotate them thereby minimizing the number of images potentially lost at any given time if the camera is lost/stolen. By creating a new folder each time you change the backup card you can keep track of which images are on which card.
Of course if you follow the advice of others and carry more hardware to back up to another device, then all of the above is not necessary.
Regardless of which method you choose, risk from theft/loss is best minimized by keeping multiple copies in separate locations. SD cards are a mixed blessing. They take little space and weigh next to nothing so are easy to store/transport. But they are also easy to lose. On Amazon you can find plastic card holders that hold multiple SD cards and fit in the credit card slot in your wallet. That makes it very easy to keep a copy on your person at all times.