I don't think it's recommended to run two motors off the same driver, which it sounds like you're trying to do. To make your configuration work, you'd have to run one forward and the other in reverse, off different drivers, at the same speed. Steppers aren't reversed by wiring them backwards, like regular DC motors; you need to change the Direction bit. Or you could give yourself a break and get one motor that's strong enough to do the job...