  We have fabricated a device composed of two closely coupled two-dimensional
electron systems, one of which resides within an AlAs quantum well (QW) at the
X point of the Brillouin zone (BZ), while the other is contained at the
$\Gamma$ point of the BZ in the alloy Al_{0.10}Ga_{0.90}As, grown directly
below the AlAs QW. The electronic properties of these two systems are strongly
asymmetric: the respective cyclotron masses in the AlAs and the
Al_{0.10}Ga_{0.90}As layers, measured in units of the free electron mass, are
0.46 and 0.07, while the effective electron g-factors are approximately 8.5 and
0. With the help of front and back gates, we can confine mobile carriers to
either or both of the two QWs, as confirmed by magnetotransport measurements.
