Cut sweet potatoes and apples into 1/4-in. slices. Place sweet potatoes in a Dutch oven; cover with water. Bring to a boil; reduce heat. Cover and simmer for 3-4 minutes or until crisp-tender; drain. Place in a greased 13x9-in. baking dish; set aside.
In a large skillet, saute apples in 2 tablespoons syrup and 2 tablespoons butter for 4-5 minutes or until crisp-tender. Layer over sweet potatoes; sprinkle with pecans.
In a small bowl, combine the cornstarch, orange zest, cinnamon, salt and nutmeg. Melt remaining butter; whisk in orange juice and remaining syrup. Stir into cornstarch mixture until well blended. Pour over pecans.
In a small bowl, combine brown sugar and cinnamon. Cut in butter until mixture resembles coarse crumbs; stir in pecans. Sprinkle over the casserole. Cover and bake at 375° for 45 minutes. Uncover; bake 15-20 minutes longer or until bubbly.