Cook squash: With the tines of a cooking fork or paring knife, pierce squash in several places. Place whole squash in a baking dish (about 9" square). Bake at 375 degrees until tender when pierced; about 45 minutes.
Meanwhile, cook lamb and beef in large frying pan over medium high heat stirring often, until meat is browned. Add onion and garlic, cook, stirring often, about 5 min. until onion is soft. Drain off fat. Stir in tomato sauce, tarragon, basil, oregano and pepper into meat. Season to taste with salt. Reduce heat and simmer, uncovered about 15 min., stir often.
Cut cooked squash in half lengthwise, then scoop out and discard seeds. With a fork, shred out half the squash into a shallow 3 quart baking dish (13x9). Spread squash with half of the meat mixture, and over that half of the parmesan cheese and half of the jack cheese. Repeat layers.
Bake casserole, uncovered, in a 350 degree oven until heated through, 25 - 30 minutes. Let stand 5 - 10 minutes before serving.