Recently, polarization conversion based on the equivalent magnetic field of Weyl semimetals (WSMs) has gained significant attention. Based on a twin WSM-layer structure, we transplant the concept of twist from Twistronics to obtain a dynamically tunable polarization converter with multifunctions and nonreciprocity. One-way conversion of linear polarization to its orthogonal state can be obtained and tuned by twisting the converter. Moreover, nonreciprocal conversion from linear polarization to elliptical polarization (with ellipticity linearly tuned from -1 to 1 by twisting the converter) in one way and to quasi-linear polarization in the other way can be obtained. Calculations show that the converter achieves 95% efficiency, high optical isolation (100 dB), and low insertion loss (<2 dB). The proposal may find applications in tunable and nonreciprocal devices for integrated photonics.