Cook linguine according to package directions. Meanwhile, in a saucepan over medium-high heat, bring broth to a boil. Add onion and garlic. Reduce heat; cook uncovered for 5 minutes. Stir in spinach; cook for 2 minutes. Add milk, cream cheese, salt and pepper; stir until cheese is melted. Drain linguine and place in a serving bowl. Add sauce and toss to coat. Sprinkle with Parmesan and mozzarella cheeses. Toss to coat.