Im taking Os this year. In dc motor, the split ring commutator turns with the coil and rubs against TWO carbon brushes.
There is O level qn in the TYS, asking why the carbon brushes must NOT be allowed to touch both halves of the split-ring at the same time.
Now, doesnt this contradict?
http://electronics.howstuffworks.com/motor6.htm
Last bullet point, your answer is all there....
I believe the question is trying to ask why ONE carbon brush cannot be touching both split rings at the same time.
The answer is in twincat's link, "Each time the commutator hits the point where it flips the field in a two-pole motor, the commutator shorts out the battery (directly connects the positive and negative terminals) for a moment. This shorting wastes energy and drains the battery needlessly"
There you hve your answer! Also, the reason why there is a 'split' in the commutator is to ensure that current will constantly travel in the same direction in the coil, making the coil turn in only one direction (clockwise/anti-clockwise).
Hope this helps. =D