  We investigate analytically, the stability of Bloch waves at the boundary of
the first Brillouin zone in two coupled Bose-Einstein condensates confined in
an optical lattice. Contrary to the single component case, we find here two
critical density regimes which determine the stability of the Bloch waves.
Breakdown of Bloch oscillations appear when n1/n2<Nc1 or when n1/n2>Nc2, here
Nc1 and Nc2 are some critical values of n1/n2. There is an intermediate regime
between Nc1 and Nc2 where the Bloch oscillations are stable and the condensates
behave like single particles.
