Sega Master System / Mark III / Game Gear
Press x12 at the title screen, then hold on controller two, and then press on either controller.
Press x4 at the title screen, then press on controller two.
Press x14 at the title screen, then on either controller to start the game with all weapon upgrades (wide beam, longer range, and faster shots).
See the main page for Quartet / Double Target - Cynthia no Nemuri (ダブルターゲット シンシアの眠り) / Double Target (더블 타케트)