I'll try to explain what JenniferMI means by the wallpaper method.
When you hang wallpaper, in order to get a perfect seam, you lay the right edge of one strip of paper (call it the "top" paper) over the left edge of another (call it the "bottom" paper) and cut through BOTH pieces. You then lift up the right edge of the top paper, remove the small strip of paper that was cut off the bottom paper, lay the top paper back down, and you have a smooth seam where the 2 strips meet. (I'm not sure if that explanation makes any more sense or not!
So with this cake, if you didn't cut the same shape out of the white that you do the blue, the blue would stick out further than the white (because it would be laying on top of the white). To make them both flush against the cake, you put the white on the cake, then put the blue over it, then cut the wave/pattern through BOTH the blue and the white which makes the line where they meet exactly the same on both pieces, lift the blue and remove the white that's under it, smooth the blue back down, and now the white and blue are both flush against the cake. To hide the seam between the 2, glue a rope over it.
I don't know if that's the way this cake was done, but it's what Jen means by the wallpaper method. I sure hope that helps!!