I was doing a bit of scooping around the the MIUI website and forums and came across this comment. As you can see, the author seems to be CyanogenMod, commenting that the next version of CyanogenMod ROM's may have a dual boot feature, enabling you to choose between 2 different OS' during start up your device.
However no one has confirmed this and it could just be a troll. You never know though.
It would be great if we could use 2 OS' on our Android devices, the possibilities would be endless.