Arrange racks in bottom and top thirds of oven, then heat oven to 450°F. Place chicken, rosemary, and garlic on rimmed baking sheet, then add lemons to center of sheet. Brush chicken with 1½ tablespoons oil and sprinkle with ¾ teaspoon each salt and pepper; roast in top third of oven until golden brown and instant-read thermometer registers 160°F, 35 to 38 minutes.
Meanwhile, on second rimmed baking sheet, toss squash and apples with remaining 1½ tablespoons oil and ¼ teaspoon each salt and pepper; arrange cut sides down. Roast on lower oven rack 15 minutes. Flip and continue roasting until golden brown and tender, 5 to 10 minutes. Sprinkle with parsley and serve with lemony chicken.
