Wide-voltage-range circuit can achieve either a high energy efficiency by operating at near-threshold voltage or a high processing performance at super-threshold voltage region. In this paper, a dual-rail voltage switching strategy is proposed for fast and efficient dynamic voltage and frequency scaling at 0.5-to-1.1V voltage range and 7.35-to-100MHz frequency range. To provide more available V/F levels in a wide operation range, the voltages of two alternative power rails are dynamically adjusted. The voltage of the unconnected power rail is adjusted to the subsequent required level in advance when the current task is processed. Besides, PMOS header device is customized for the dynamic dual-rail scheme. The proposed strategy is applied to a wide-voltage-range processor. Based on the simulation results, the switching time between two voltage levels is within 10ns. Furthermore, the proposed strategy enables multiple V/F levels with the significantly reduced energy consumption by up to 61.2% as compared to the previously published method.