Lots of people now have 2 phones*, most commonly a Blackberry and iPhone. Why not have 1 case that can hold them both.
There would only need to be a thin spacer between the screens to stop them knocking together, rather than the large flap that some cases have.
The case would swing all the way open so that the phones are back-to-back when making calls, which would look odd to passers by.
There could be other case variants to allow other combinations of phones or a modular approach so that the 2 halves of the case can easily and seamlessly be conneccted and removed.
*I have a $30 Nokia.-- marklar, Jan 11 2012 [21 Quest] I know lots of people who have Nokias or Androids, but have 1 phone. I also know several people who have both a Blackberry and an iPhone.
// putting all your eggs in one basket// I agree, that is a concern.-- marklar, Jan 11 2012 Maybe you need three. (pockets, phones, pads, etc...) One for when the other two go missing.-- blissmiss, Jan 11 2012 *wonders how many batteries it takes to power a 7 foot tablet...-- RayfordSteele, Jan 11 2012 random, halfbakery