Isotopic fractionation of mono-substituted N2O species by the stratospheric sinks, photolysis and reaction with O(1D), has been investigated previously by atmospheric and laboratory measurements. Here, we report on photolysis of the most abundant poly-substituted isotopologue, 15N15N16O. The associated fractionation constant equals the sum of individual fractionation constants at central and terminal nitrogen sites. Based on statistical considerations, a more general additivity rule is proposed that extends to the remaining poly-substituted N2O species and is compared to predictions from two theories. Atmospheric mixing effects do not lead to measurable deviations from random isotope distributions since variations of isotope ratios are too small. Fractionation by the O(1D) sink likely also follows the additivity rule. Consequently, the stratospheric 15N15N16O abundance is expected to reflect its statistically dictated value. The value of 15N15N16O as a potential new tracer of stratospheric processes may thus be limited, but this remains to be verified by actual measurements.