Hi Robert,
Sorry I think we are talking about two different things, I'll try to make my english good enough to be understood.
First, you cannot add into the same ProjectSuite two projects that shares the same Sources\Files, even if you just add Files into a genuine Sources. This is a huge limitation of TC to my opinion, but that's not the point.
I was saying that, using a Project that have added "as existing items" Stores\Files, you cannot add files in it. It says that the file is added, but when you save and exit, you could notice that the file has been physicaly copied to the folder but not added in Files.tcFiles.
You have to open the Project that have genuine Stores\Files to be sure that the file has been taken into account.