Hello, it is obvious, indeed: drag and drop the layer from the layers panel to the second document...
You can also select the move tool, hold down ALT and drag and drop the layer to the second document, from the file itself.
If you hold Shift, the layer will be centered (if the files do not have the same dimensions) of it will land at the same location (if they do)
Or select all (Ctrl+A), Copy (Ctrl+C) and Paste (Ctrl+V) in the second document.
Yet another solution is to right-click the layer in the layers panel, choose "duplicate..." and set the destination as your other document.
Hope this leads you in the correct direction!
OK. I finally got it. I almost -never- open documents undocked. And apparently this only works if the documents are free floating. Once I thought to undock them, it is indeed easy.
I guess I had thought/hoped there was a more intuitive way or rather, just a straight key command that wouldn't require undocking.
But THANKS! THANKS! THANKS!