Combine the first 6 ingredients in a 13 x 9-inch baking dish. Add 1/2 teaspoon salt. Make 3 diagonal cuts 1/4-inch deep across cut side of each squash and zucchini half. Place squash and zucchini halves, cut sides down, in baking dish. Marinate squash and zucchini at room temperature for 15 minutes.
Remove squash and zucchini from marinade, and discard marinade. Place squash and zucchini on grill rack coated with cooking spray. Grill 5 minutes on each side or until tender. Sprinkle evenly with 1/4 teaspoon salt.