Airport operator BAA explained the trial was designed to minimise disruption and will see the threshold for triggering emergency systems at the airport lowered.
At present the two runways are restricted to take-off or landing respectively, switching each afternoon, except in an emergency.
Under the terms of the trial, both runways can be used when a plane faces a ten-minute wait to land or take off or if 30 per cent of all flights are delayed by more than 15 minutes.
The average time delay for flights at the west London airport is currently 12 minutes.
BAA added the number of flights at Heathrow would remain capped at 480,000 aircraft movements a year.
The trial finishes on February 29th.
A second will run from July 1st 2012 until September 30th.
The trial was recommended by the department for transport and will be overseen by the Civil Aviation Authority.
