If you are running JUST ONE fan per circuit (meaning one fan->one pot you will need a pot or rheostat of at least 18 ohm and 18W. You'll probably only find a rheostat in that spec. If you run the two after one pot, you will need a 9 ohm 36W rheostat at least. Do not skimp on either the resistance (you won't get down to 6V) or the wattage (you'll overheat and fail) or you'll be an unhappy camper.
Here is a 15 Ohm 15W:
http://www.allelectronics.com/cgi-bi...-15&type=store
This may work, getting you to around 7-8V without an overheating problem.
Meh, that was the only quick candidate. Mebbe you'll have to order two of them?