  The spectra of plasma and magnetoplasma excitations in a two-dimensional
system of anisotropic heavy fermions were investigated for the first time. The
spectrum of microwave absorption by disk-like samples of stressed AlAs quantum
wells at low electron densities showed two plasma resonances separated by a
frequency gap. These two plasma resonances correspond to electron mass
principle values of $(1.10 \pm 0.05) m_0$ and $(0.20 \pm 0.01) m_0$. The
observed results correspond to the case of a single valley strongly anisotropic
Fermi surface. It was established that electron density increase results in
population of the second valley, manifesting itself as a drastic modification
of the plasma spectrum. We directly determined the electron densities in each
valley and the inter-valley splitting energy from the ratio of the two plasma
frequencies.
