  By using different widths for two AlAs quantum wells comprising a bilayer
system, we force the X-point conduction-band electrons in the two layers to
occupy valleys with different Fermi contours, electron effective masses, and
g-factors. Since the occupied valleys are at different X-points of the
Brillouin zone, the interlayer tunneling is negligibly small despite the close
electron layer spacing. We demonstrate the realization of this system via
magneto-transport measurements and the observation of a phase-coherent, bilayer
$\nu$=1 quantum Hall state flanked by a reentrant insulating phase.
