I was heavily reliant on SplitView on macOS. Due to the new status bar in Dorico and a (relatively?) new minimum horizontal window size constraint in Preview on macOS, SplitView no longer works when you have a small screen.
Therefore, my suggestion is to use a small app called SlidePad. This way, you can have a PDF floating above a full-screen Dorico and easily switch between them. It’s useful for moments when you don’t have a second monitor (or if you find it faster to work on a single monitor).
Other suggestions are welcome, of course, but this is what seems to be the quickest for me after a day of experimenting.