It depends on the medium that is used to print the enamels. You can print on a previous layer as soon as the enamels are dry, or they have been fired.
I use water based enamels, which take a long time (days) to dry, so I do fire in between layers of printing. If a fast drying medium is being used, you can print subsequent layers of imagery directly on top of one another. However, you do have to be extra careful that you do not scratch or damage the previous printed layers, as they are delicate until fired onto the glass.