A method is described in which the interference of radiated second-harmonic electric fields generated by a pair of oriented molecules intercalated into double-stranded DNA is controlled and measured. The results show that the relative molecular orientation of the two molecules significantly changes the magnitude of the observed second-harmonic generation intensity, which is described by a simple model that accounts for the interferences of the radiated fields. The technique presented shows promise for future experiments investigating structural changes induced by the formation of a DNA-biomolecule complex.