Don't know, but my Renault Espace has two fans alongside each other in front of the radiator. They both come on together when temp gets about 3/4 up the gauge. I guess there isn't room for a big enough single fan to do the job.
Maybe just for more efficient cooling? I know of a company called Kenlowe that manufactured after-market twin cooling units for modded cars such as Ford Escorts etc.
not always for air-con, they are often mounted side by side, seperate air con fans have one in fron and behind the rad, Vauxhalls call them pushing or pulling fans