  We have studied conditions of photon Bose-Einstein condensate formation that
is in thermodynamic equilibrium with ideal gas of two-level Bose atoms below
the degeneracy temperature. Equations describing thermodynamic equilibrium in
the system were formulated; critical temperatures and densities of photonic and
atomic gas subsystems were obtained analytically. Coexistence conditions of
these photonic and atomic Bose-Einstein condensates were found. There was
predicted the possibility of an abrupt type of photon condensation in the
presence of Bose condensate of ground-state atoms: it was shown that the
slightest decrease of the temperature could cause a significant gathering of
photons in the condensate. This case could be treated as a simple model of the
situation known as "stopped light" in cold atomic gas. We also showed how
population inversion of atomic levels can be created by lowering the
temperature. The latter situation looks promising for light accumulation in
atomic vapor at very low temperatures.
