I think most of the engine heat (barring exaust heat) is dissipated through radiator(my guess its around 90%) and only abt 10% is radiated by engine body itself. now, the reason for that is though the total surface area of radiator is abt same as that of engine itself, it is effectively multiplied a number of times due to circulation of water and air flow rate passing through radiator.
Now, the extra fan should only improve 10% part of engine cooling and not the major heat source ie radiator. Unles, as suggested by FLi_GTi, we increase air flow rate through radiator by installing second fan ahead of it. Moreover, i am not totally comfortable with the idea of hot air pockets in engine compartment because its not air tight compartment and hot air has lot of free space to exit it.
Also, this is what i tried today. I let my car idle while parked untill its fan turned on. At that time there was strong flow of hot air under the car body at driver door level. U guys can try it too.
BTW, i am pretty impressed by the result of that modification and cant wait to see a pic.