Place squash in a saucepan with boiling water to cover. Cook over medium heat 10 minutes or until tender. Drain; cool.
Cut a 1/4-inch lengthwise slice from each squash; scoop out, and reserve pulp, leaving shells intact. Mash pulp, and set aside. Remove ends of squash.
Sauté reserved pulp, onion, and carrots in 1 tablespoon butter in a large skillet until tender. Remove from heat; stir in pimiento, 1 cup stuffing mix, sour cream, salt, and pepper. Spoon mixture into shells.
Place stuffed squash in a lightly greased 13- x 9- x 2-inch baking dish. Combine remaining stuffing mix with remaining butter; sprinkle evenly over each squash. Bake at 400° for 10 minutes or until thoroughly heated.