Cut each squash lengthwise into 1/4-inch slices, cutting to within 1 inch of stem end. Cut tomatoes into 1/4-inch slices.
Combine chopped basil and next 4 ingredients in a large bowl, stirring until blended. Add squash and tomato, tossing gently to coat; let stand 1 hour.
Remove vegetables from marinade, reserving marinade; insert tomato slices between squash slices, and secure with wooden skewers.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 8 minutes on each side or until crisp-tender, basting often with reserved marinade. Garnish, if desired.