  Bosonic thermal transport through a two-level system is analyzed at
temperatures below and comparable to the two-level energy splitting. It is
shown that in the low-temperature regime transport is dominated by correlated
two-boson processes analogous to electron cotunneling in quantum dots under
Coulomb blockade. We present a detailed analysis of the sequential-cotunneling
crossover and obtain essentially an analytic description of the transport
problem. Perturbative analysis is complemented by employing scaling properties
of the Ohmic spin-boson model, allowing us to extract an anomalous low
temperature scaling of thermal conductance.
