We introduce an architecture for changing the polarization state of far-infrared through submillimeter radiation that employs two Martin-Puplett interferometers. One interferometer is oriented with its beam-splitting grid at an angle of 22.5 degrees with respect to the Stokes Q axis. The second is oriented with its beam-splitting grid at an angle of 45 degrees. By modulating one of the arms of each interferometer, it is possible to arbitrarily adjust the polarization state that a polarization-sensitive detector measures when placed at the output of the device. Because of this flexibility, one application of this device is as a calibrator for a polarimeter. In addition, it is conceivable to use such a device as a modulator for a far-infrared/submillimeter polarimeter. As such, this system has several advantages over a half-wave plate. First, the capability to measure circular polarization will provide the instrument with a novel method for checking systematic errors, as the circular polarization of most astronomical continuum sources is expected to be near zero. Second, such a device is easily adapted to work at different wavelengths, thus facilitating the construction of far-infrared and submillimeter polarimeters with multiple passbands. Finally, the small linear throws necessary for modulation eliminate the need for complicated systems of gears and low temperature bearings that are common in wave plate systems and often prone to failure. We present a Jones matrix analysis of this modulator architecture and compare the performance of this device with that of a half-wave plate.