The heat transfer and fluid flow patterns in the mixed convection regimes for the double circular cylinders arranged transverse to the vertical air stream are presented. The difficulty of the non-singular grid generation for the present triply-connected region has been effectively resolved by the FEM-FDM grid blending technique which requires embedding of finite elements in the small area of major geometrical difficulty. The unsteady streamfunction values are assigned rigorously on the cylinder surfaces and on the far-field boundary. It has been found that the Karman vortex street breaks down behind the double heated cylinders in a transient manner for a certain Richardson number range due to the buoyancy effect and vortex interaction, in contrast to the sudden breakdown applicable to a single heated cylinder.