Lightly oil lollipop molds. In small bowl set over saucepan of simmering water, stir white chocolate just until melted and smooth. Remove bowl from over water. In another small bowl set over same saucepan of simmering water, stir bittersweet chocolate just until melted and smooth. Cool chocolates slightly. Drizzle small spoonful of white chocolate into bottom of each mold. Drizzle small spoonful of bittersweet chocolate over. Using toothpick, swirl chocolates slightly to marbleize. Repeat layering of chocolates and swirling until molds are filled. Insert lollipop sticks into groove in molds and rotate sticks to coat with chocolate. Gently tap molds on work surface to release air bubbles. Refrigerate lollipops until very firm, at least 3 hours or overnight. Refrigerate cookie sheets until chilled. Invert molds onto chilled cookie sheets. Gently bend corners to release lollipops (it may be necessary to let molds stand 30 seconds and then repeat bending). If desired, insert each lollipop into cellophane bag and tie decoratively with ribbon. (Lollipops can be prepared 3 days ahead. Cover tightly and keep refrigerated.)