Didn't know that was a possibility, so thanks for the prompt reply.
Tried Ctrl-Alt-Down and it inverted my image on screen--am replying standing on my head
I was able to change the keys assigned and got some results. "Next file" goes to the next file as listed on the spine, not restricted to files already open in the editor. BUT in a Eureka moment I thought of changing the order of the files in the browser so that the two files I need to work with were one after another and Voila you have given me a quick way to toggle between them. Thank you, Thank you, Thank you.