You develop both concurrently, or at least I do.
My Analog Rytm plugin/editor source code looks like this:
Analog Rytm Editor
- Plugin
- Source
- Standalone
Plugin contains a plugin specific build; and gets everything else from Source (this build contains all the bits and bobs for AU / VST etc.)
Standalone contains a specific standalone build; and gets everything else from Source (There is no AU / VST stuff at all in this build)
The Source folder contains all the shared code; components, data model, engine etc.
All UI/UX, code changes etc., like adding a new button the GUI, adding new functionality, happen in Source; so changes are then reflected in both the Plugin and Standalone versions concurrently.