We have studied the photoluminescence of doubly stacked layers of InP quantum dots in magnetic fields up to 50T. We find that in contrast to the triply stacked layers studied previously, which were strongly coupled for layer separations of 4 nm or less, the dots in the double layers do not couple. We explain this surprising difference in behaviour by proposing a model in which the holes are confined in the GaInP layers separating the stacked dots. For triply stacked dots two such layers exist, allowing the possibility of coupling via the holes. In doubly stacked dots, there is only one such layer, thus coupling is absent. Our model is consistent with measurements of the exciton masses and radii in the dl,ts, and with calculations of electron and hole wave functions in self-assembled quantum dots. (C) 2000 Elsevier Science B.V. All rights reserved.