H Gantry" design. It sounded pretty intriguing, so I googled it. The basic idea is that two motors control a full x/y coordinate range, while not carrying the motors on the gantry. This makes the gantry more stable, since it's not having to carry the extra weight. In addition, the crossbar is supported on both ends.
commercial product that uses an H Gantry. It's apparently a well-known technique and patents have expired.
frame showing the mechanism a bit more clearly. If both motors turn at the same rate, motion is in the X axis. If only one motor turns, motion is in the Y axis. Turning both motors produces motion on both axes.