  We investigate the stability of a rectangular Josephson vortex lattice in a
stack of two long Josephson junctions. It is shown that in an infinite stack at
low velocities the lattice is unstable with respect to triangular vortex
lattice formation and at high velocities the lattice may be either stable or
unstable due to parametric resonance. This resonance may be "suppressed" in
three ways: (i) by increase of damping in the system, (ii) by the increase of
the external magnetic field, or (iii) by increase of the velocity of the
lattice. We show that the finineness of the stack may provide the stability of
the rectangular vortex lattice.
