Preheat oven to 450. In a large skillet, melt butter over medium heat; add zucchini, yellow squash, shallots, and garlic. Season with salt and pepper. Cook, stirring occasionally, until zucchini and squash are crisp-tender, 4 to 6 minutes.
Add cream, and cook until thickened, about 5 minutes. Remove skillet from heat; stir in 1/2 cup panko and cup Parmesan.
Spoon mixture into a shallow 2-quart baking dish. Sprinkle with remaining panko and Parmesan; season with salt and pepper. Bake until top is golden, 8 to 10 minutes.