Heat a grill pan over medium-high. Coat pan with cooking spray. Thread tomatoes onto 2 (8-inch) skewers. Drizzle 1 tablespoon oil over tomatoes and onion wedges; add to pan. Cook 4 to 5 minutes on each side or until tomatoes are blistered and onions are lightly charred.
Remove tomatoes from skewers; place in a medium bowl. Add onion wedges, remaining 1 tablespoon oil, olives, and remaining ingredients; toss.