It sounds a bit strange to me : as you can state, I defined 2 circles S1 and S2. The first one is the full domain where I want to compute the FEM, the second one is the object which produces a thermal effect. “Thdeuxdim”, that is “TT2d” in your message, is the mesh with these 2 objects.
If I follow your logic, I do not understand how the computer should “know” that my boundary condition is to be applied on the bottom face of my second object, and not on the bottom face of both (that is : including the full domain) ?