If they are iced with royal icing, then yes techincally it is possible that they can sit on top of each other without pillars as the icing should be strong enough to support the weight. if the cakes are iced with fondant icing then the icing alone will not be able to bear the weight. You can however buy dowelling like plastic rods which go through the cake and so are not visible they help to take some of the weight and then the cakes can be stacked. I have bought these from a cake decorating shop, but they maybe available in Hobbycraft as I know they stock some cake making stuff.