In large skillet, over medium heat, saute garlic and onion in butter and oil about 2 minutes. Reduce heat, and layer zucchini and yellow squash slices on top of onion and garlic. Add salt and pepper, cover and steam 10 minutes, or until squash is tender.
While vegetables are steaming, bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Add the crushed tomatoes to the zucchini mixture, stir and heat slightly, but do not cook the tomatoes. Serve the zucchini sauce over the pasta.