  Current mm/submm interferometers, like the Atacama Large mm/submm Array
(ALMA), use receivers that register the sky signal in a linear polarization
basis. In the case of observations performed in full-polarization mode (where
the cross-correlations are computed among all the polarization channels) it is
possible to reconstruct the full-polarization brightness distribution of the
observed sources, as long as a proper calibration of delay offsets and leakage
among polarization channels can be performed. Observations of calibrators,
preferably with some linear polarization, with a good parallactic angle
coverage are usually needed for such a calibration. In principle,
dual-polarization observations only allow us to recover the Stokes $I$
intensity distribution of the sources, regardless of the parallactic angle
coverage of the observations. In this paper, we present a novel technique of
dual differential polarimetry that makes it possible to obtain information
related to the full-polarization brightness distribution of the observed
sources from dual-polarization observations. This technique is inspired in the
Earth-rotation polarization synthesis and can be applied even to sources with
complex structures.
