Canonical has released a new Ubuntu developer preview version for Nexus devices which has an option to dual-boot with Android. Switching between mobile OSs are now easy. On each side, an app with a simple user interface will enable you to boot back and forth at the tap of a button. See the image below:
Note: Dual boot is not a feature suitable for regular users currently. It is recommended to be installed only by developers who are comfortable with flashing devices and with their partition layout.
