Yes, you can. You have to be careful as you may see a small layer line when you hold the casting up at the right angle between the first and second pour. The best thing to do is to pour the first layer but rotate the mold so the resin coats the walls of the mold. Then simply allow to cure. The layer on the walls will help break up the perfectly flat line between the first and second layer. It also helps to pour the second layer as soon as you can over the first layer once the first layer has gelled. This will ensure the best bond and a chemical bond/crosslinking between the two layers.