The Other Half is nothing more than NFC, with or without assisted bluetooth connection. The devices comprising an Other Half, would need to fit well within a Bluetooth profile.

While it does limit what can be put on the device, the subtle combination of bluetooth and nfc in this case, with an easy to deploy form factor and case, provides a very compelling, if small and subtle idea on how to both use the unused space at the back of a phone, and how to do things like aesthetic customization, small and useful peripherals, etc.. Bluetooth, NFC, flash storage, and small low voltage microcontrollers provide a very powerful base on which to build all sorts of widgets.