Place 2 (12-ounce) cans of frozen apple juice concentrate in a 10-inch nonstick skillet over high heat and cook until the juice comes to a boil and foamy bubbles cover the entire surface, about 10 to 15 minutes. Caution: Once the juice starts to foam, the bubbles will rise above the level of the skillet. Immediately reduce the heat to medium, and continue to cook until the juice is reduced to 1½ cups, about 20 minutes more. Let cool, then place in a covered container and refrigerate.
Before using the syrup in a recipe, remove it from the refrigerator as you start your preparation, so it will have time to come to room temperature. The syrup will be thinner and easier to pour and measure.