For the impatient, the answer is we would make both devices active participants for display and input. Like two mathematical musicians playing a duet, the beautiful music they create can equal more than two (perhaps it can go as high as eleven).
OK. Confession time: we do not have an Apple Watch or a new iPhone and we did this research before they were announced. We used devices that are publically available. But, that should not make this research any less interesting.
With the premise that two tools working together can create greater values, the User Interface group at Autodesk Research, with partners at the University of Toronto and Carnegie Mellon University, started to explore these possibilities:
The watch has an acceleronmeter in it so it can provide intital information about how the hand it is attached to is working as an input device. Specifically, what is the orientation of the hand relative to the phone. Knowing the orientation of the hand means a person is not limited to the traditional finger tip press. People could now also enter data with:
What could this do for reading email? You could have one finger touch point for navigation (move through message, go to next message, etc.), one finger touch point for email management (archive, delete, etc.) and one finger touch point for things like cut, copy and paste.
Of course, a person is not limited to entering data on the phone. Wearing the watch on the inside of the wrist, so that the watch screen is oriented in the same manner as the phone screen in the hand, a person could be gesturing across devices:
And of course you could tap the phone and watch together or tap and flip devices to initiate additional commands.
Knowing a phone and watch are paired, could improve security - you can only use the phone if your watch matches. Plus, a new gesture could be made for unlocking the phone.
This is where it really gets cool!
What if you used the watch as a tool to zoom in on a map without losing your position?
What if you used the watch as a tool palette for the phone?
New gestures and connections open up a lot of possibilities. You can read more about this research in the Autodesk Research paper entitled Duet: Exploring Joint Interactions on a Smart Phone and a Smart Watch as well as watching the movie below. What kind of things could you imagine doing with these abilities?