For a large class of Kramers-Fokker-Planck type operators, we determine in the semiclassical (here the low temperature) limit the full asymptotic expansion of the splitting between the lowest eigenvalue (0) and the next one. In a previous work with Herau and C. Stolk we did so in the case when a certain potential (or exponent of a Maxwellian) has preciely one local minimum and then the splitting is "large". In this new work we treat the case when the potential has two local minima. Then the splitting is exponentially small and related to a tunnel effect between the minima. Our most direct source of inspiration has been works by Herau-F.Nier, B.Helffer-Nier, but our methods are quite different.