Add the flour and salt to a large mixing bowl.
Spread the tallow onto a sheet of parchment or plastic wrap in an even layer. Cover and freeze for 10 minutes. Once firm, use a knife to cube both the butter and tallow. Add it to the flour.
Use your fingers to incorporate the butter & tallow into flour, continue until the dough looks shaggy. Add in the water and apple cider vinegar. Stir together until the water is absorbed.
Dump the dough onto a clean surface and gently knead the dough just enough to form into a ball. Using a bench scraper, cut the dough in half and stack one on top of the other, press down. Repeat this lamination process 2-3 more times.
Divide into 2 equal sized discs and wrap in plastic. Refrigerate for at least 12 hours and up to 2 days. When ready to use, remove from the fridge and let it sit for 10 minutes before rolling out, Enjoy the flakiest pie crust ever!