It seems that instead of copying one layer of the sandwich at a time, you can shorten the process as follows:
1. in the sandwich file, select all layers
2. Choose Edit > Copy Special > Copy Merged
3. In the table file, select Edit > Paste As New Layer
This assumes you do your layer work (duplicate, move etc.) in the sandwich file.
Please see attached file for the 600 square version.
Notice when the file was saved using File > Save As menu, the transparency of the two plate file is removed. I saved as a JPG file.
Overland Park, KS