Beyond the ideal detailed balance evaluation, layer-dependent thermal radiation conversion has been systematically investigated here for tandem device consisting of a GaSb top subcell and a 0.53 eV GaInAsSb bottom subcell. Relying on the experimentally-accessible material parameters, it is demonstrated here that tandem device in its N-on-P configuration displays a superior thermal conversion efficiency, and the proper doping profile should be controlled as Nd = 7-9 x 10(17) cm-3 and Na = 5 x 10(17) cm(-3) for GaSb subcell while Nd = 6-7 x 10(17) cm(-3) and N-a = 3.5 x 10(17) cm(-3) for 0.53 eV GaInAsSb subcell. Moreover, due to different subcelllimited performance outputs, the dependence of thermal conversion efficiency on radiator temperature shows a remarkable "S-shape" feature, and the open-circuit voltage of tandem cell can be up to 0.7-0.8 V for typical radiator temperatures. Finally, limited by the larger bandgap of GaSb material, superior thermal conversion efficiency can be always expected for tandem cell when comparing to GaSb cell, but only for radiator temperature larger than 1500 K when comparing to 0.53 eV GaInAsSb cell.