In this video I will show you how to use a new tool that was just released a couple of hours before, it is called CoolBooter and it basically allows you to have two iOS versions at the same time installed on any Jailbroken 32-Bit device. Yes, no signing is required, this means that if your iPhone 5 or 5C is jailbroken, you can have both iOS 9.3.3 and iOS 6.0.1 for example, but on iPhone 4S, 5 and 5C you can also go back to iOS 8, iOS 7 and so on.
The 32-Bit iPhones are: iPhone 4, 4S, 5 and 5C.
This downgrade / dual-boot is untethered which means that you don't need any computer to boot your device if you restart it. This tool also enables you the Verbose Boot and you can also Jailbreak the new iOS you install.
This is currently not compatible with 64-Bit devices (starting from iPhone 5S) because kloader app (used in the tool) is for 32-Bit only, but expect it to be ported in the future.
Both iOS versions are separated, they don't interact / share files one with each other and they are both fully functional (you can place calls, send SMS, connect to WiFi etc).