Two horns a few metres apart, pointed at the same patch of sky, make an interferometer. As a source drifts across the beams, the path length to each horn changes. The two signals move in and out of step. Their combined power rises and falls in a fringe pattern. The distance between the horns sets the fringe spacing. That relationship makes this arrangement useful.
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Correlation and the Two-Element Interferometer - Where the fringes come from. Covers convolution, autocorrelation, and cross-correlation. Explains resolution limits for a single dish, adding and multiplying arrangements, and the effects of finite bandwidth. Comes with a notebook you can run.
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Additive Interferometry Using Two DSPIRA Radio Horn Telescopes - LightWork Memo 31, by John Makous. Derives additive interferometry from the path length difference. Describes the two-horn system tested at Green Bank and its solar transit on a 5.0 m baseline. The measured fringe spacing was 0.043 rad, compared with the predicted 0.042 rad. The ending mentions signal multiplication as work the group had completed but had not yet written up.
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Setting Up a 2 Horn Interferometer is the practical companion. It covers equipment, wiring, east-west alignment, and operation of the adding or multiplying spectrometer.