Well the best answer to that is to find out how many amps the AC fan draws. The find out how many the regular fan draws. Look at the circuit and see what gauge wire is feeding power to the fans. Look at the amp rating on the fuse and relay. Then see if the circuit would be able to support the extra load. Or you could just try it and see if it melts the wires.

I would check and see if the fan circuit wire is 10 gauge or larger and if the relay is a heavy duty 30 amp relay. If so then you should be fine. If not you take a chance of an eletrical fire. You could make your own circuit and never have to worrk about it.