In a small saucepan over medium high heat, stir together raspberries, 1 tablespoon agave syrup, 1 tablespoon water, juice of 1/2 the lemon, and 1/2 teaspoon cornstarch.
In another small saucepan over medium heat, stir together blueberries, 1 tablespoon agave syrup, 1 tablespoon water, juice of the other 1/2 of the lemon, and the other 1/2 teaspoon of cornstarch.