The stretching-mode (SM) absorption of isolated SeH^- or SeD^- molecular defects in alkalihalides show a resolved spectral structure due to the different reduced masses of the six natural stable Se isotopes. In a FTIR study of these defects in KCl and KBr, we have observed new SM absorptions of defect pairs which scale quadratically with the defect concentration. For `equal pairs' SeH<sup>-</sup> -SeH<sup>-</sup> and SeD<sup>-</sup> -SeD<sup>-</sup>, the fundamental SM absorption shows (in contrast to `mixed' SeH^- -ScD^- pairs) a distinctly different Se-isotope splitting, while the overtone splitting of all pairs remains similar to that of the isolated defects. Besides a simple classical coupled-oscillator treatment, we present a quantum-mechanical calculation, explaining the observed spectral behavior by resonance interaction of the molecules in pairs of equal (or `nearly equal') frequencies.