  Tunneling of two particles in synchronous and asynchronous regimes is studied
in the framework of dissipative quantum tunneling. The critical temperature T_c
corresponding to a bifurcation of the underbarrier trajectory is determined.
The effect of a heat bath local mode on the probability of two-dimensional
tunneling transfer is also investigated. At certain values of the parameters,
the degeneracy of antiparallel tunneling trajectories is important. Thus, four,
six, twelve, etc., pairs of the trajectories should be taken into account (a
cascade of bifurcations). For the parallel particle tunneling the bifurcation
resembles phase transition of a first kind, while for the antiparallel transfer
it behaves as second order phase transition. The proposed theory allows for the
explanation of experimental data on quantum fluctuations in two-proton
tunneling in porphyrins near the critical temperature.
