The two primary specs to be concerned with when matching a motor driver with a motor are current and voltage.
For current, look at the stall current rating of the motor you are using, and make sure the motor driver is rated for at least that current (continuous rating, not peak). The driver can be higher than the motor current, but you do not want the motor stall current to be higher than the rating of the driver.
In your case, when wiring two motors to a single driver, you'll want to make sure the driver is rated for 2x the motor current, since it is driving two motors.
For voltage, you want to make sure the operating voltage of the motor is within the range of voltages the driver will work with. You don't have to worry about how many motors the driver will be driving. 2x 12v motors will still be operating at 12v, not 24.
You might want to revisit http://en.wikipedia.org/wiki/Series_and_parallel_circuits
If you have two motors connected up to a battery (let's just ignore the concept of an h-bridge/driver right now) in series, each will see half the voltage, but draw the same current. If they are both connected in parallel to the battery, they will see the same voltage, but draw twice the current.
This is, in fact, how the 12 volt PowerWheels ride-on toys implement a dual speed system; these toys have two motors (one for each rear wheel connected thru gearboxes), and an very ingenious (and robust - and patented) switching system that connects the motors to the 12 volt SLA in either "series" (low speed) or "parallel" (high speed).
So - for the OP - if you connect the motors in series to your motor driver/h-bridge - each motor will only use half the voltage (and thus turn slower), but will only draw the same current as a single motor:H-Bridge Positive ----> +Motor1 --- -Motor1 ---->+Motor2 --- -Motor2 ----> H-Bridge Negative
In this case, your h-bridge needs to be rated for the stall current of a single motor, and for the voltage ratings of the motor. Note that should one of the motors in a serial configuration slow down or stall, the other motor will tend to speed up (in the case of a PowerWheels, this acts as a crude form of an electronic differential). I would actually arrange for the motors on each side of the DF4WD to be connected in serial, but you might want to experiment.
If you connect the motors in parallel to your motor driver/h-bridge - each motor will use the same voltage (and turn at the same speed - within reason), but will draw double the current:H-Bridge + ----> +Motor1 ----> +Motor2
H-Bridge - ----> -Motor1 ----> -Motor2
In this case, your h-bridge needs to be rated for the total stall current of both motors, and for the voltage rating of a single motor. If one of the motors in the parallel configuration slows down or stalls, the other motor continue to run at the same speed (assuming your power supply is up to snuff, of course).
In both situations, as jraskell
noted - you need to match your h-bridge to your motors.
Something you might want to think about, though - is to use an h-bridge per motor, and control each with the Arduino; if you can monitor the running current of each motor, you might be able to detect slip vs. stall, and use that information to slow-down or speed-up the motor - a form of traction control. Another option would be to mount Mecanum wheels (http://en.wikipedia.org/wiki/Mecanum_wheel
) to each motor, and drive them independently to allow for vector translation of the platform (which I suspect is the reason for the provision of independent motors on this platform).