This isn't what they're proposing. What they're saying is that Android will be on the touchscreen, and Ubuntu will available on the main screen.
Presumably, Ubuntu apps will be given access to Android's data store and events, but Ubuntu apps won't run on the touchscreen, and vice versa.
For example swiping, pinch to zoom, etc. Many apps use a swipe to the left or right to perform and action. How would this work with a mouse?