It has to be done on the Target Mobile App/Website as far as I know. I couldn't find a place to do it using my laptop.
I've figured it out now. Just go to https://m-secure.target.com/giftcards/my-cards, log in there, then follow the directions. It's not intuitive, but I had to enter the gift cards first, click on one of them, then I can combine it with others.