As long as the first layer has partially cured, you can pour the next layer. This will minimize the lines between layers. If you are worried your resin may not be curing properly, keep a little bit in your cup to check on it without bothering the resin in your mold.
When using EasyCast resin, you can make the layers as thick as necessary, but mix no more than 16 ounces of resin and hardener (total) at once.