[JerryF504]

If your y-harness connects the batteries in series it will double the voltage. Two 6 cell packs in series will give you 14.4 volts and 4200mahs. If it connects them in parallel it will double the amperage. Two 6 cell packs connected in parallel will be 7.2 volts and 8400mahs. The speed control is referring to cells run in series so if it says it can do 12 cells safely that means it can do 14.4 volts, so you should be ok.