I want to make a cake that has an 8" cake sitting on a 12" cake. My first question, is what is the correct terminology to describe that? Is it tiered or stacked or separated? I'm so confused!
Second, what size plates do I need? I don't want them to show because I want the top cake to look as if it's sitting directly on top of the bottom cake. Also, do I need cardboard cake circles if I'm using separator plates?
That would be a 2-tier, stacked cake. You do not need separator plates at all, only cardboard. You will need something in the bottom tier (wooden dowels, bubble tea straws, etc) to support the upper tier.
Do you mean there will be no border around the bottom of the 8" cake?
To make the board invisible, cut it from a piece of 3/16" white foamboard, about 1/4 larger than the diameter of the BAKED CAKE. Measure your cake, not your cake pan.
Place your cake on the board. Then, place the board on a larger, greaseproof board as a work surface. (You may need to use a blob of icing or some double-stick tape to keep your cake board from sliding around on the work board.)
Ice the cake AND the board it is on, all the way to the bottom. The board disappears!
Pop the cake in the freezer (on it's work board) for about 10-15 minutes before you try to move it to the 12" cake. This will ensure the icing is nice and firm and less likely to get marked up.
Once the 8" is on the 12", you can smooth around the bottom of the 8". If there's a gap, you can pipe a small ring around the bottom of the 8" and smooth it against the cake.