As a new feature targeting only developers at this point, Ubuntu Touch can now dual-boot onto Android devices. Dual-booting Android and Ubuntu Touch still requires flashing the device and modifying the partition layout. Upstream Android is supported, the open-source AOSP Android, and CyanogenMod are all supported for this Ubuntu Touch Phone/Tablet dual-booting.
Changing the boot operating system isn't done from an option at the boot-loader but is handled by an app in both Ubuntu Touch and Android for handling the switching of operating systems at next boot. The initial Ubuntu Touch dual-boot installation is carried out via an Android app install. More details can be found via the developer.ubuntu.com site.