The attached sketch is for 5 individual bookcases that will be attached to the wall and each other. My question is about the two 6′ cases. I am adding a middle panel since I cannot have a shelf going that wide. Since all the units are attached to each other, do you think I should add a fixed shelf to stiffen the center panel? If they were to stand alone, I would do that, but since they are all attached I didn’t think it was necessary and it would allow all the shelved to be adjustable. Thanks for you input.