Sound and light 'waves' can interfere with each other. They may do this constructively (superimpose) or destructively (cancel out).
Think about something like a sporting event. If one person sings alone then they wouldn't be heard very well. If you have an entire ground singing all those sound waves come together and can be heard kms away. All the extra energy is coming together and causing the vibrations of the air particles to be even stronger (this is what sound basically is).
If you brought two tv sets together they would be twice as loud only if the waves of vibration were 'perfectly' in phase.
Your two torches probably wouldn't be twice as bright but would more then likely be brighter. The only way to look at light sources and see how they interfere is to use lasers. These are light sources where the waves are in phase with each other.
Check out anything on Young's two-slit experiment or on diffraction gratings to explain it.
A quick google gave this:
http://www.physicsclassroom.com/Class/light/u12l1b.html
it's not a bad explanation.
Quite a strong argument for light being a wave as opposed to a stream a particles. (It is quantum mechanics that makes the link but that is a different story)