Do you do this very often? I had a cake this weekend that measured out to be exactly what I needed if I baked a 12 x 18 and then cut it apart and reassembled 2 of the pieces with buttercream.
It seemed like a good plan until I came to the part where I had to glue the 3 x 6 pieces together to make a 6" square top tier. It didn't seem very stable and then I was wondering if the customer would be able to tell I had put 2 pieces of cake together...
Can they tell when they cut the cake? It was chocolate cake with vanilla buttercream.
I've done some carving like that before. What I do is after I "glue" the pieces together, I trim the edges to make them even and I ice the whole thing super smooth so that there is no "tilt" to top of the cake...you can't tell after its cut!
I hope that helps!