On the 737 the difference in rejected take off distance with both reversers operating and only one reverser operating, is only ten feet on a dry runway. Depending on what caused the fire warning, it is worthwhile remembering that any fuel leak spraying around will atomise because of the reverse thrust plume and intensify the blaze. That is what happened with the British Air Tours (?) 737 at Manchester.
It would be illogical to deliberately reverse an engine on fire simply because it is easier to pull two reverse levers than one.