Sam the most imp thing to remember is support, adequate support is needed to make sure that the cake layer that is stacked does not sink into the layer underneath. For this dowels made of plastic or wood are available at bakery supply shops. For light cakes plastic straws can also be used.
Dowels are available at shops that stock baking supplies, they are hollow plastic/ wooden sticks to give support, you need to drive 4-6 into the cake and top it with the second tier alongwith the cardboard board so that there is firm support. Additionally you can drive in a dowel right through both tiers for additional security.