This question probably answers itself: In the kit there are (2) pieces for the Coming back. Are they bonded together before inserting in postion similar to the Deck beam? The manual is not clear on this point.
I only recall one coaming back when building my MC 16.5. Drawing #3 shows one 6mm part and drawing #4 shows one 6mm part in the top view. I would guess to say that as long as you have a thickness of at least 6mm you should be fine.
I took lots of photos during my build but have not had time to edit and post. I will be ready to varnish and paint as soon as it warms up enough to do so. Here are a few...