

Don’t break your fingers when braking.
But seriously two fingers on the brakes is good in many circumstances as it reduces reaction time and depending on your bike, setup and your ability you may be able to maximise braking with just two fingers.
Shorty levers can help.
Covering the brake with two fingers can reduce reaction time which is useful especially in traffic.
On the track it is easier to use two fingers to trail brake and maintain throttle control and you will notice many of the best riders using two finger braking.