The High Sierra version of the App Store that can run in Mojave. According to my tests they can run in parallel, downloads work, access to purchased tab works.
You can easily access old macOS downloads.
It doesn't need to modify anything on the system.
You might have to open the Terminal and
xattr -r -d com.apple.quarantine <Drag and Drop Old App Store.App> in order to run it
* Note: The bundle is named Old App Store.app but it will appear as App Store