To ensure that the layers stick together, the bottom layer shouldn't be too cold, and the top layer needs to be steaming hot when you pour. The heat from the second layer will melt the top of the bottom layer every so slightly so they can fuse together. If the bottom is too cold, it will cool the liquid layer down instantly and they won't stick. A YouTube viewer also suggested that you can scratch the top surface of the bottom layer slightly to create little crevices that allows the two layers to adhere together better. They didn't specify what to scratch it with (I'm thinking tip of a paring knife) or how many scratches you need to make (I'm thinking over the entire surface), but it sounds sensible. I have not personally tried this, so if you try it out let me know how it goes!