This is not a solution to your exact question, but another option you might consider: instead of using the split screen, you can create a new window in Microsoft Word of the same document, and then position those two windows side by side.

I do this because when I want to work the way you are working, I prefer the side-by-side windows, rather than a horizontal split (I have more horizontal space than I have vertical space on my monitor, after all, so side-by-side allows me to see more of each "frame" of the same document).

So what I say is "new window", for which I have set up a custom command to open a new window copy of the same document I am working on.

Then I say "side-by-side", which I have set to position the two word documents next to each other.

Now, instead of fiddling with mouse clicks, if I want to tab between the two documents, I simply say "alt tab" and the focus will change between the two docs.

Dragon Professional Individual v15.6. Windows 10. Knowbrainer 2017.