The theory of patterns formation for a reaction-diffusion system defined on a
multiplex is developed by means of a perturbative approach. The intra-layer
diffusion constants act as small parameter in the expansion and the unperturbed
state coincides with the limiting setting where the multiplex layers are
decoupled. The interaction between adjacent layers can seed the instability of
an homogeneous fixed point, yielding self-organized patterns which are instead
impeded in the limit of decoupled layers. Patterns on individual layers can
also fade away due to cross-talking between layers. Analytical results are
compared to direct simulations