:-)
WATER temps
As a second radiator will only lower your water temps, we need to know your current water temps. A passive rad after an active rad will lower your temps by only 0.5 degrees or so.
(unless it is real large, or your temps are real high)
Why not place a 12 cm fan on 7v on there? this will help a lot, and can lower your temps with 2 or 3 degrees. (depending on your current water temp and size of the additional radiator)