[greedyspectator]

@Zero
If the lasers have the same frequency, then they will overlap due to resonance. This does increase power compared to a single laser... but then you could have a single laser with twice the power and twice the pulsing and have an even more effective laser (with better penetration and surface deposit). Single 1000 joule laser < Double 1000 joule laser < Single 2000 joule laser. Resonance is basically what a laser does even before firing a laser beam, the laser apparatus arranges so that all the electrons in the laser medium has the same greatly increased energy (thus an overlap in frequency), and when the electrons jump back to their normal levels, the extra energy is released.

EDIT: And lasers have to be pulsed, otherwise the damage they do is not enough. When a high energy photon hits a target, part of the target is vaporized, which will block of and bloom other incoming photons. If you pulse and time the laser correctly, hopefully the first pulse will vaporize part of the target, and the second pulse will arrive just after the vapor dissipates in the air, and the third pulse after that... you get the idea. Even continuous lasers have to be pulsed at least a microsecond apart.

And lasers are not supposed to be visible in the air, unless the laser energy is so great that it actually heats up the air visibly, and small dust particles in the air reflect the photons to such an extent. There are some observatories that fire a visible laser to the atmosphere as an astronomical tool, but note that these lasers are made to be inefficient since they aren't weapons, but observation apparatus.