I’m not sure I understood fully the use of key-pairs.
1. Only sign the “DLL-project” with a key file for the reason that the latter assembly of which it becomes part, will “recognize” its DLL.
2. So when I use two or more “DLL-projects” in my solution/assembly, I sign each of them with a key file.
3. In fact, I could use the same key file, every time I sign a “DLL-project”. And when I later integrate one or more of my signed “DLL-projects” in a solution, the compiler does the remaining

Yes, you can reuse the same key. There would be no issues I can think of you would have to do. Bryan