Unfortunately, as I understand it, doing that would simply short out the power supply, not the motor. Turning both lines high at the same time shorts out the power supply. Braking shorts out the motor itself. For brake control, you need access to each quarter of the bridge individually and you activate the lower two halves at the same time, connecting to motor leads together. But since the upper half is not turned on, the positive end of the power supply is disconnected.
Turning both on at the same time for any length of time is a very bad idea.
Is that what you meant, or am I misunderstanding?