  A system of two quantum wells (QW), one made of HgCdTe and the other of
HgCdMnTe, subjected to electric and magnetic fields $F$ and $B$ parallel to the
growth direction, is proposed and described theoretically. It is shown that in
such a system the spin $g$ factor of mobile electrons strongly depends on the
sign and magnitude of electric field. Adjusting $F$ at a constant $B$ one can
transfer almost all electrons into one or the other QW and polarize their spins
along the desired orientation. Changing $B$ at a constant $F$ can produce a
similar transfer and polarization effect. Possible applications of the spatial
reservoirs filled with spin-polarized electrons are discussed.
