Leonid Nemirovsky, of the Better Than Nothing AutoLISP site, shares his method for placing one layout tab in an AutoCAD file into more than one sheet set list.
"If you need to use the same layout in more than one sheet set, you can create a copy of the layout for use in another set. For example, if you have a layout called General_PLAN that's used in a sheet set created for a preliminary project, you can't also use it in a sheet set for the final stage. To get around that restriction, open the file with the General_PLAN layout in it, create a copy of that layout, give it a new name (such as General_PLAN_Final), and use that copied layout in the new sheet set."
Notes from Cadalyst Tip Patrol: I have found this limitation in AutoCAD's sheet sets to be frustrating. There have been times when I wanted to use a single layout tab of a file in more than one sheet set or sublist of a sheet set, but I couldn't do it. Len's solution will work just fine, but if another user isn't aware that one layout is simply a copy, then changes to the drawing might not get made to both the copy and the original. A second approach is to copy the source file, using one for preliminary use, and using the copy for the final submittal of the project.