Comment 18 for bug 1070770

Shawn, you need MultiROM of at least version 7 and after having installed the Ubuntu ROM as one of the alternative ROMs, before you boot Ubuntu for the first time, you have to go in to the ROM list in Recovery, select the freshly added Ubuntu ROM and in the menu for modifying it, select the item to disable flash-kernel. The kernel_kexec_42.zip is only for Android and you have to apply it to the original Android system, by the "Install ZIP" entry of the main menu of Recovery. After having done all that you are able to boot both systems. Boot Ubuntu and follow the steps for the initial configuration. Kernel updates pulled by updating the system will not break the boot sector or the Android kernel and when you reboot into Ubuntu you will get the new kernel which got installed by your update. So you will be able to also test the kernel and make use of all our fixes, and boot back to Android whenever you want.

See also

https://wiki.ubuntu.com/Nexus7/Installation#Having_both_Android_and_Ubuntu_installed_for_dual_boot