Preheat the oven to 375 degrees. Butter or oil a 3-quart baking dish.
Place the carrots and sweet potatoes in a steamer set above 1 inch of boiling water, and steam for five to 10 minutes, until just tender. Drain and toss with the remaining ingredients in a large bowl. Combine well, and scrape into the prepared baking dish.
Place in the oven, and bake 40 to 50 minutes, stirring every 15 minutes, until the sweet potatoes and carrots are thoroughly tender.
Dot the top with butter, and bake another 10 minutes until the top is lightly browned.