Fill a 6-quart pot three fourths full with salted water and bring to a boil. Plunge lobsters headfirst into water and boil, covered, 7 to 8 minutes. Transfer lobsters with tongs to a colander and drain head down to remove excess liquid. Finely chop shallot. In a small saucepan melt butter with shallot, tarragon, vinegar, and salt and pepper to taste. Arrange lobsters on a platter and using kitchen shears remove thin, hard membrane from underside of each tail by cutting just inside outer edge of shell. Spoon some tarragon butter over each lobster tail and serve remainder on the side. Serves 2.