As part of a feasibility study for a far-infrared Fourier-transform spaceborne spectrometer, the criteria that drive the choice of the instrument configuration have been identified as broadband operation, dual input and output ports, optics of the interferometer with full tilt compensation, and measurement of both planes of polarization of the source on a single detector. Despite the fact that some of these requirements are apparently difficult to reconcile, a new configuration of the polarizing interferometer that meets all the above requirements has been identified. The considerations that led to the design of this new configuration are discussed.