Bob, As David noted, you can use one decoder but it must handle the current draw from both motors. The Athearn motors of old drew quite a bit of current, up to an amp each. Not sure what the stall current would be. With the drive broken in and well lubed, and the unit not weighted down to where it won't spin its wheels, you probably could use a decoder rated at two amps. Should be plenty of room! While on the topic of two motor locos, some articulated steam has two motors, with these you can use two decoders so that you can simulate the wheel slip which often occurs when starting. Simply start one set of drivers a little before the other, using both knobs on the Digitrax throttle to control the motors. With sound, the effect would be awesome.
Gary
Gary