Definitely go for two - it's clearly the most stable option. You haven't stated how wide your gantry is though, if it's really small and light cuts you might get away with it.

You should only use one stepper motor per driver, not two on one driver. It might be ok...as long as the motors are identical, but what if one stalls? Another way to do it is to link the two ballscrews with a timing belt and just use one motor.

It is normally done with two motors connected to two drivers connected to separate outputs on the parallel port. Mach3 will, using limit switches, control both so that both drivers receive the same signal whilst the machine is running, and when homing it can move them individually to square up the gantry. The manual for Mach3 explains it - look up slave axis.

Yet another method is to use wires to link the two sides of the gantry mechanically. Think of how it's done for the ruler on a drawing board...