Following is a link to a fan speed control that can handle 5 Amps of fan motor(s) that may be large enough:
http://www.lutron.com/instructions/033039c.pdf
The fan control should exceed the amp load of the fans plus 25% . If you dont know how much your fans draw, check some of the fans at the store you purchase the speed control from to assist in your calculations.
You will need to leave both fans on high with the builtin switches, and control with the speed control.