Apple - Open two views of the same PDF

Open the first one regularly and then use the open command with the -n option.

1. Open Terminal
2. Type open -n and drop the file into the Terminal window. The full path of the file will appear after what you've typed.
3. Hit enter.

The -n option opens a new instance of the application (in this case the default one, as no other is specified) even if one is already running.

From the Print dialogue choose the PDF button, Open PDF in Preview.

The temporary copy seems just to vanish when closed, as you would want; and it has the same name and location as the original. If you markup the new copy, it creates a duplicate with new name and you choose to save or not.

Skim has menu items for marking a page and going back to a marked page. You can assign shortcuts to them in System Preferences.