In bowl stir together oil, rosemary, and 1/2 tsp. salt. Add potatoes; toss.
For charcoal grill, arrange medium-hot coals around drip pan. Place grill wok, basket, or foil pan directly on rack above drip pan; heat 5 minutes. Add potatoes; cover and grill 10 minutes, stirring once. Add squash; cover; grill 15 minutes more, until potatoes are tender, stirring once. Transfer to serving dish. Sprinkle with remaining fresh rosemary. Serves 4 to 6.