I wouldnt advise this. The fans draw a lot of current and to put 2 fans in the circuit will draw twice as much on the same wiring At best you will find the fans run slower because of the wires cant feed them enough current. At worst the wires will get so hot the whole car will catch fire.

Use the feed to the standard fan as a relay trigger to drive the second fan. Take the power feed for the relay directly (or as near as possible) from the battery (fused of course!).