Cut neck off each squash. Peel necks and cut crosswise into generous 1/3-inch-thick slices. Overlap slices in 2 large baking dishes.
Sprinkle with salt and pepper. Spoon 6 tablespoons tangerine juice over squash. Spoon cream over squash; dot with butter.
Bake squash until beginning to brown, about 30 minutes.
Mix tangerine peel into remaining 10 tablespoons juice; drizzle over squash.
Place 1 sage leaf atop each squash round.
Bake until squash are very tender and tops are browned in spots, about 30 minutes longer (do not turn squash over at any point). (Can be prepared 4 hours ahead.
Let stand at room temperature. Before serving, heat in 375°F oven until warm, about 15 minutes.)
