  We study polymer-polymer phase separation in a common good solvent by means
of Monte Carlo simulations of the bond-fluctuation model. Below a critical,
chain-length dependent concentration, no phase separation occurs. For higher
concentrations, the critical demixing temperature scales nonlinearly with the
total monomer concentration, with a power law relatively close to a
renormalization-group prediction based on "blob" scaling arguments. We point
out that earlier simulations and experiments have tested this power-law
dependence at concentrations outside the validity regime of the scaling
arguments. The critical amplitudes of the order parameter and the zero-angle
scattering intensity also exhibit chain-length dependences that differ from the
conventional predictions but are in excellent agreement with the
renormalization-group results. In addition, we characterize the variation of
the average coil shape upon phase separation.
