To receive different polarizations, different circuits and receivers must be used for LCP and RCP. Simply adding voltages out of phase does not do the trick, since it simply creates a new polarization vector on the sky. To get polarizations, intensities must be measured.
References
Berge, G. L.; Muhleman, D. O.; and Orton, G. S. "High Resolution Interferometric Observations of Venus at Three Radio Wavelengths." Icarus 17, 675-681, 1972.
Thompson, A. R.; Moran, J. M.; and Swenson, G. W., Jr. Interferometry and Synthesis in Radio Astronomy. New York: Wiley, 1986.
