manual J program

As to putting a second drop down in the same cell, I believe that the answer is no, at least I have never done that nor seen it done.
As to putting materials lists in their own column, I feel that this would make things easier to work with now and easier to make additions in the future.
For example, with Exterior materials listed in column AA (Sheet19) the source for the drop down in cell F8 (CONSTRUCTION DETAILS sheet) could be changed to:

Formula:

=OFFSET(Sheet19!$AA$2,0,0,SUMPRODUCT(--(Sheet19!$AA$2:$AA$10<>"")))

This would reduce the need for the step of producing a named range (Exterior) and allow for new materials to be added to the list in cells AA5:AA9
Let us know if you have any questions.