One month after the first signs of Linux support on several iPhones appeared, with the Linux 4.4 kernel (and Android!) finally booting on the iPhone 7 and some other models, well-known "jailbreakers" and developers @Ralph0045 and @mcg29_ have published instructions on how to dual-boot 64-bit iOS devices on their GitHub page.
These instructions go in-depth to explain how to partition the root filesystem, create custom ramdisks and finally boot into the devices.
While these instructions are currently primarily aimed at iOS partitions, thanks to the low-level nature of the procedure Linux and Android kernels could be probably started with (almost) the same ease.
Combine this with having proper Linux kernels already booting (although still without flash storage and some other important drivers), and we can (not too boldly) predict dual-boot Linux support arriving in a not so remote future. So, iOS users, you can prepare yourselves to show off your triple-boot iOS, Android and Ubuntu Touch iPhones.
Source: @ralph0045 / Twitter