Great questions! While I can’t guarantee that it will completely eliminate lines, pouring the second layer of resin while the first layer is still curing will minimize them. You are correct in that you will need to do it while the first layer of resin is in the gel stage. This is when the resin is starting to cure, but not solid. If you were to place a toothpick in the resin at this point, it will be a thick, and perhaps chunky, liquid. The time for this to occur varies with every resin. Unfortunately, at least the first few times, you are going to have to babysit your resin to see when this occurs. In my experience, it happens within the ‘double the pot time’ range. For example, if the pot time of the resin is 30 minutes, the gel time will likely happen within 60 minutes. The gel time usually only happens for a short period of time, so you are going to have to watch it closely and be ready to pour your next layer.
As for pouring only 1/2 inch at a time, that also depends on the resin. Some have recommendations on thickness of pours, while for others, it’s more about the volume.