  Starting from the Fisher-Kolmogorov-Petrovskii-Piskunov equation (FKPP) we
model the dynamic of a diffusive system with two mutually communicating
identical patches and isolated of the remaining matrix. For this system we find
the minimal size of each fragment in the explicit form and compare with the
explicit results for similar problems found in the literature. From this
comparison emerges an unexpected result that for a same set of the parameters,
the isolated system studied in this work with size L, can be better or worst
than the non isolated systems with the same size L, uniquely depending on the
parameter $a_{0}$ (internal conditions of the patches). Due to the fact that
this result is unexpected we propose a experimental verification.
