My bad with explaining. The two laser example was to try to give an easy example and I ended up confusing everyone I guess. You have a single laser (for each wavelength usually). The laser shoots and is being returned (leaf). The same impulse is being returned again but from another surface at a marginally later point in time (the ground). If you would visualise that with common LIDAR software, you would have two points on the same trajectory: one at e.g. 20 m height above zero (ground) and one at 30 m above zero (leaf). You only have a single laser available that penetrates leaves but not ground. I hope that makes it more understandable. English is not my first language as well, so in case you want me to explain something again, I can try again or look up resources in English!