Preheat oven to 350°F. Take butter and smear it on the bottom and the sides of a 13x9 inch pan. Set aside.
Add the chopped yams to a large pot. Fill with water. Bring to a boil and cook until the sweet potatoes are fork tender. Drain the water.
Transfer the cooked sweet potatoes to the prepared 13x9 baking dish.
In a medium sized saucepan add the apples and 4-5 cups of water. Place the saucepan over medium heat. Bring to a boil and then turn the heat to low and simmer for 5 minutes or so, or until the apples are fork tender. Drain the water from the apples, reserving 1 1/2 cups of the apple water. Scatter the cooked apples overtop the cooked yams.
Take the 1 1/2 cups of apple water and add it to a small saucepan. Add the frozen orange juice concentrate to the water. Stir to combine. Next, add the brown sugar, sugar, cornstarch, salt, orange zest, and butter. Place the saucepan over medium low heat and cook until it thickens slightly, should be the consistency of syrup. Remove from heat.
Pour the sauce overtop the yams and apples. Cover the baking dish with aluminum foil.
Bake for 30 minutes at 350°F, or until warm and heated through.
Note: You can make this dish a day in advance. Follow the recipe up until step 6. Rather than placing the foil covered dish in the oven, place it in the refrigerator. The following day you can pull it out of the fridge and bake it for 45 minutes or until heated through. You have a longer bake time because it's coming straight from the fridge. We love to make it a day prior because the flavors deepen!