It is theoretically possible but is not something that has been implemented so far. There are two main issues: 1. It will require repartition of the device. Android encrypts the largest partition known as userdata, so it can not be used to store the alternative OS and a separate partition needs to be created like it was done on Gemini PDA. 2. Bootloader modifications to provide minimal boot menu and ability to boot from alternate boot partition (at least for dual-boot).