The photon model of light may be the most frequently over-applied model in physics. Lamb expresses my opinion fairly clearly here:
The photon concepts as used by a high
percentage of the laser community have
no scientific justification.
In my experience, many physicists who answer simple questions about matter without unnecessary reference to quarks or gluons are incapable of answering simple questions about light without unnecessary reference to photons.
This is puzzling, because very few experiments are capable of distinguishing between the existence and nonexistence of photons.
If you are genuinely interested in the photon model of light, then be prepared to do an awful lot of math to predict even fairly simple experimental results. You will, of course, be using a more correct model, but one should use the right tool for the right job.
If, however, you're interested in the experimentally observable behavior of light, then Maxwell's equations will give you the right answer in the vast majority of cases. For example, you ask if two different lasers can interfere. They can! See this question:
Is it possible to observe interference from 2 independent optical lasers?
I'm sure the photon model predicts this result, but I suspect not without a fairly strong grasp of the math. If you'd never heard of photons, and all you knew were Maxwell's equations, this result isn't very surprising.
I'll close my answer with a question: For what type of experimental prediction is the photon model actually relevant? For what types of predictions is the photon model confusing, misleading, or more effort than it's worth?
Examples so far:
The photon model is relevant to:
The photon model is not relevant to:
My last two claims are intentionally bold. Prove me wrong!