  By means of analytical calculations and numerical simulations we study the
diffusion properties in quasi-two-dimensional structures with two exciton
subsystems with an exchange between them. The experimental realisation is
possible in systems where fast and slow exciton subsystems appear. For
substantially different diffusion coefficients of the species the negative
diffusion can be observed, if one measures the transport properties of only a
single subsystem, just as was obtained in experimental studies for
quasi-two-dimensional semiconductor systems. The initial transition from a fast
subsystem to a slow one results in a delayed release of fast excitons in the
area close to the original excitation spot. Hence, the signal from the fast
excitons alone includes the delayed replenishment from the transition of slow
quasi-particles. This is seen as the narrowing of the exciton density profile
or decrease of mean-squared displacement which is then interpreted as a
negative diffusion. The average diffusion coefficients for the combined
population are analytically expressed through the diffusion coefficients of
fast and slow excitons. Simple analytical expressions for effective diffusion
coefficients obtained in limiting cases are of interest both for theoretical
and experimental analysis of not only the exciton transport, but also for a
variety of systems, where fast and slow moving subsystems are present.
