A critical issue in biaxial tensile tests is that the central area of the cruciform specimen does not deform to a desired level when the specimen has fractured in other areas. Contrary to the central area thickness reduction approach adopted by the bulk of researchers, this study introduces a thickness-increased sandwich specimen for large strains in the central zone to evaluate the formability and hardening behaviors of sheet metals under complex forming situations. The initial cruciform specimen is strengthened outside the central area by attaching a metal plate in similar shape to each side. This novel specimen has the ability to avoid the inherent characteristics of materials being altered in a thickness-reduced specimen during the material-removing process; moreover, it is applicable to thin sheet metals. Finite element simulations are used for parameter optimization. The optimum set of three key parameters: fillet (R) between the arms, radius (r) of the central zone, and line angle between the notch edge and the horizontal axis (theta), appear to be R = 5.0 mm, r = 4.0 mm, and theta = 21 degrees. The optimized design is corroborated using a new testing device with better synchronization on which various displacement ratios of 1:0 and 0:1 can be implemented by applying 90 degrees wedges so that plane strain conditions in the central zone can be achieved. A maximum equivalent plastic strain of approximately 15% is achieved in the central zone, indicating the effectiveness of the strengthened specimen in increasing the plastic deformation in the desired zone.