Preheat oven to 375 degrees. Line a 13- x 9-inch pan with heavy duty foil and place the pork in the pan.
In a small bowl combine garlic, olive oil, sage, salt,and pepper. Rub the mixture all over both tenderloins. Bake in the preheated oven for 20 minutes.
While pork is cooking, place sugar, balsamic vinegar, and soy sauce in a small saucepan over medium low heat. In a small bowl, make a slurry with the water and cornstarch and add to the saucepan. Heat, stirring occasionally, until mixture begins to bubble and thicken slightly. Spoon a small amount of the glaze on each tenderloin during after the first 20 minutes of the cooking time.
Return pork to the oven and continue to cook for an additional 15 minutes or until the internal temperature of the thickest tenderloin has reached 150 degrees F when tested with an instant read thermometer. Remove it from the oven, tent the pan with foil, and allow it to rest for a few minutes.
Slice each tenderloin into 1-inch slices and return them to the foil-lined pan with the pan juices. Warm the remaining glaze slightly over low heat and pour over sliced pork. Stir a little to mix with pan juices.