1 cup (or more) canned low-salt chicken broth
3 pounds rutabagas, peeled, cut into 1-inch pieces
1/4 cup (1/2 stick) butter
Preheat oven to 400°F. Arrange butternut squash in single layer in 13x9x2-inch glass baking dish. Add 1 cup broth. Sprinkle with salt and pepper. Cover tightly with foil. Bake until squash is very tender, about 45 minutes. Drain squash. Transfer squash to processor.
Meanwhile, cook rutabagas in large pot of boiling salted water until tender, about 20 minutes. Drain. Add to squash in processor. Process until mixture is smooth, adding more broth if necessary.
Transfer squash and rutabaga pureée to heavy large saucepan. Add butter. Stir over low heat until heated through, about 5 minutes. Season with salt and pepper. Transfer purée to bowl. (Can be prepared 1 day ahead. Cool. Cover and chill. Rewarm over low heat.)