Is it possible to use a split rudder as an air brake?
It is possible but there are side effects.

If both rudders would be deflected to opposite sides to cancel out each others yaw moments, their drag will act as an air brake. But since the top rudder is further from the center of gravity than the bottom rudder, the resulting roll moments of the rudders don't cancel each other out. As a result the aircraft will start to roll. This can then be counteracted with the ailerons. 

Since the rudders don't deflect as far as air brakes would, they would be less effective. If they would be designed to deflect more, the increased forces upon them and on the control system would require a heavier construction and thus add weight to the aircraft.