Heat oven. Heat oven to 200°F.
Mix sugar and water. Mix sugar and water together until sugar is damp throughout (this is easiest with your hands so you can really tell if all the granules have gotten wet).
Press into molds. Press mixture into the mold taking time to press really well into all the crevices. Firmly pack it in as hard as you can.
Flip out skulls. Very carefully invert the mold onto a parchment lined baking sheet, letting the sugar skull slip out.
Bake until firm. Bake skull to harden the sugar for 10 minutes. Let cool completely.
Stick sides together. Pipe royal icing on the back of both of the skulls then press firmly together. Hold about a minute to make sure they stick. Let dry, then decorate.