Hey Kitchen Research! And thanks, Gammy—I was going to link to the same article :)
Yes, you can certainly double this recipe to use a 9x13 pan. Just keep in mind that the shortbread will be a little thicker and that you'll need to bake it for longer—I'm guessing closer to an hour, but just check in on it frequently toward the end and look for the same visual cues (when the top is "deeply golden-brown and dry to the touch").
Here you go... an article right on Food52 with all you need to know about pan conversions: https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size . Looks like a 9"x13" has a bit less area than 2x the recipe. You could either lose a bit of the dough to make them the same thickness or use all the dough and the squares would be a bit thicker, which I think would be fine.
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Yes, you can certainly double this recipe to use a 9x13 pan. Just keep in mind that the shortbread will be a little thicker and that you'll need to bake it for longer—I'm guessing closer to an hour, but just check in on it frequently toward the end and look for the same visual cues (when the top is "deeply golden-brown and dry to the touch").