The same data directory — and that's the default when it first asks you when you install whichever version you install second. (In the future we may just remove that prompt.)

When it's properly configured, you won't be able to open the Zotero pane in both simultaneously, and Zotero for Firefox will automatically switch into a lightweight connector mode (similar to the other browser plugins) when Standalone is opened and switch into full mode when Standalone is closed.