  The phase separation instability occurring with increasing nearest-neighbor
repulsion V in a two-band Hubbard model (CuO chain) is discussed. Quantum Monte
Carlo simulations indicate that this transition is associated with a
level-crossing if the filling fraction is close to 1 (half-filled lower band).
Spin-density-wave fluctuations then dominate before phase separation.
Superconducting fluctuations dominate only at considerably higher doping
levels.
