  We calculate the tunneling magnetoresistance (TMR) of
Fe$\mid$ZnSe$\mid$Fe$\mid$ZnSe$\mid$Fe (001) double magnetic tunnel junctions
as a function of the in-between Fe layer's thickness, and compare these results
with those of Fe$\mid$ZnSe$\mid$Fe simple junctions. The electronic band
structures are modeled by a parametrized tight-binding Hamiltonian fitted to
{\it ab initio} calculations, and the conductance is calculated within the
Landauer formalism expressed in terms of Green's functions. We find that the
conductances for each spin channel and the TMR strongly depend on the
in-between Fe layer's thickness, and that in some cases they are enhanced with
respect to simple junctions, in qualitative agreement with recent experimental
studies performed on similar systems. By using a 2D double junction as a
simplified system, we show that the conductance enhancement can be explained in
terms of the junctions energy spectrum. These results are relevant for
spintronics because they demonstrate that the TMR in double junctions can be
tuned and enhanced by varying the in-between metallic layer's thickness.
