I made a wedding cake for a friend with 2 x 12 inch cakes stacked on top of each other and it was fine. Each one was cut in half so there were 4 layers essentially. It was chiffon cake so it was quite tall, probably equivalent to 3 victoria sponge cakes height wise. The only 'issue' was that each portion would have been ridiculously big if it was the whole height of the cake so I used a clear cake board in between the 2 cakes so that they could be separated for serving. The board was about an inch smaller than the cake size- it doesn't have to be clear but as it was a wedding cake I thought it would be nicer for photos etc if it didn't show.
If you want your cake to be cut the whole way through maybe add a couple of dowels to the centre so it stays in place while you're icing it etc. Also, make sure you have a very long palette knife if you are icing it!