I don't know if you'll notice a change in mileage. The mechanical power of the belt driven fan will have to be replaced with the extra drag on the alternator making electrical power for the added fans. Were you thinking of making the fans run full time or turned on by a thermostatic switch? Is your air conditioning condenser in with the radiator or on the side? If it is cooled by the radiator fan, it might need to be switched so it runs when the A/C is on.
Just some things to think about.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI