You want to see the main Openbox view on one screen and the preview spreadsheet on another. To do this:

1. In the Openbox window, click “Help and settings” then “Openbox settings”
2. Click “Other”
3. Make sure the box “Show preview in new window (requires restart)” is ticked
4. Press OK.
5. Close and restart Excel

When you next create the preview it will appear as a separate window to the main Openbox window. This means you can move it around, including to a second (or third) screen if you have one.