  In In_xGa_{1-x}As/n-GaAs double quantum wells (DQWs) containing an electron
gas, the magnetoresistance (MR) peculiarities under parallel magnetic fields
caused by the passing of the tunnel gap edges through the Fermi level are
revealed. Peculiarities positioned in high fields (~30 T) can only be explained
if the spin-splitting of the In_xGa_{1-x}As conduction band is considered, that
was neglected in the GaAs/AlGaAs heterostructures, for which solely the effects
of this nature have been observed so far. In Ge/p-Ge_{1-x}Si_x DQWs containing
a hole gas, local MR peculiarities under parallel fields are discovered as
well. But the tunnel gap in these DQWs is too narrow to be responsible for
these observations. We suppose, they are due to a complicated shape of the hole
confinement subbands.
