Metasurfaces are advantageous in wavefront manipulation owing to their compact and flat nature. Particularly, ultrathin and completely smooth metasurfaces with giant phase delay and perfect impedance match are critically required for practical applications. Here, we propose an ultrathin and holeless metasurface composed of simply a pair of membranes. This metasurface supports duo unity transmissions with completely conjugate phase shifts occurring at two extremely close frequencies. This allows the metasurface to present giant phase delay and endow with high refractive index (n = 18) when the wave penetrates through. Such a property is employed to control the wavefront of acoustic waves to realize planar lens focusing, negative refraction, negative reflection, and directional emission. The proposed design principle of acoustic metasurface provides promising avenues for acoustic wave manipulation and may enable extensive applications in beam steering, acoustic imaging, energy harvesting, and surface waves.