Well, I finally got it, just before I saw your last posting. Here's how I had to do it - Open 1st file, and "tile vertically"; then WHILE STILL IN THE 1ST FILE, do file/open (on the menu bar of 1st file), click on the name of 2nd file, click "open".
What I was trying to do before was "file/open" the 2nd file from "my documents" and it would not work. I had to do it while still inside the first folder, which is probably what you were trying to tell me, but I was too "thick" to get it!
Normally when I open a file, I never use "file open" - I just double click on the file I want and it opens.