I'm assuming that the radiator manufacturer has a spec for the minimum airflow needed to operate the radiator efficiently. I'm also assuming that there's a maximum airflow, too (may or may not be spec'ed). Too high of airflow may impede the transfer of heat from the radiator to the air. Think of it this way. If you stand in front of fan at medium speed, it cools you off. If you crank the fan to max, you feel more airflow, but it doesn't cool you off any more than medium speed. My point is that depending on the design of the radiator, 2 fans may reduce it's efficiency.