Give us a bit of info about your system's fan layout. No, double-stacking the fans won't help, it may actually hurt. What will help is to use a second fan to exhaust the same amount of air that the intake fan is pushing into your case.
Another big help will be to make an easy path for the air to go in and out. Try breathing through a drinking straw and you will understand why your 120mm fan may not be working as well as it should. Cut the metal away where the fan is, drill big holes in the bezel or cut the bottm edge of it off, and you will get a lot more airflow. Add an exhaust fan to the rear if there is a vacant spot for one, or else get a case that has one if you can afford it.
You can also add a slot fan below your video card, it will help get the video card's heat production out of the case before it reaches anything else.