I believe this feature already exists, I turned it on by accident at some point.
I later worked out that the down arrow in the bottom right corner hides the bottom panel and moves the subtitles over the video (assuming the video window is wide enough).
You’ll lose the highlighting of which half of the other subtitle is currently on screen if they don’t match, otherwise it’s all the same.
Perhaps there should also be check boxes for show/hide side panel and bottom panel in the main settings dialog box so this is clearer?