Preheat oven to 400°. Cut sweet potatoes and russet potatoes into 1/8-inch-thick slices, using a mandoline or sharp knife. Toss together potatoes, 3/4 cup heavy cream, and salt in a large bowl. Transfer to a lightly greased 2-qt. gratin dish, and spread potatoes into layers. Top with 1 cup Gruyère cheese. Pour 3/4 cup more heavy cream over potatoes, and sprinkle with remaining 1/2 cup Gruyère. Cover loosely with aluminum foil. Bake at 400° for 40 minutes. Uncover and bake 20 to 25 minutes or until potatoes are tender and cheese is golden brown. Transfer to a wire rack; cool 5 minutes.