  We study two-component electrons in the lowest Landau level at total filling
factor $\nu _T=1/2$ with anisotropic mass tensors and principal axes rotated by
$\pi/2$ as realized in Aluminum Arsenide (AlAs) quantum wells. Combining exact
diagonalization and the density matrix renormalization group we demonstrate
that the system undergoes a quantum phase transition from a gapless state in
which both flavors are equally populated to another gapless state in which all
the electrons spontaneously polarize into a single flavor beyond a critical
mass anisotropy of {\bf $m_x/m_y \sim 7$}. We propose that this phase
transition is a form of itinerant Stoner transition between a two-component and
a single-component composite fermi sea states and describe a set of trial
wavefunctions which successfully capture the quantum numbers and shell filling
effects in finite size systems as well as providing a physical picture for the
energetics of these states. Our estimates indicate that the composite Fermi sea
of AlAs is the analog of an itinerant Stoner magnet with a finite spontaneous
valley polarization. We pinpoint experimental evidence indicating the presence
of Stoner magnetism in the Jain states surrounding $\nu=1/2$.
