Bassman, combining two projects as I described above does not include Screensets or any other (project specific) settings made in the second project that is being added. As you are aware, Screensets are project specific. If you want to transfer/copy/integrate the Screensets of one project into another simply use the Import Screenset option within the primary (current working project). Of course, keep in mind that you must have the other project which contains the desired Screensets also open. So, if you combined both projects via drag and drop, open the project just merged over and then toggle back to your current project and do the Screenset Import (then you can just close the older project, doing the merge does not delete/remove the older project it is still there just as it was when last saved). Easy and simple.
Hope that clarifies and improves your workflow. If not, I am still here to take your questions.
Kind regards,
tecknot
P.S. I always have "Allow Only One Open Project..." unchecked. I can see that this option would prevent accidental edits to the wrong project, but if a second project is open temporarily, I just close it immediately after I done getting what I need from it. Adding extra steps of unchecking and rechecking Preferences just adds more to your workflow which I try to avoid.