In order to teach players to keep all shots low above the net, except of course for clears. is this a good idea???
You can if the players need a visual goal, or use a piece of tape or string to same effect. Can be quite good for practicing serves to have a gap to aim for
the second net ends the rally, which is basically a strong reinforcement for what happens if you hit a shot in the kill zone.
But you'll need volleyball posts or similar to set up the higher net, a couple of rulers can be taped to the psot and a string put across, giving an easily varied target for serving etc