It can't be done. Android is open source making it possible (well, legal) to dual boot on WM phones, WM is not.
I'm sure it could be done if someone was brave (or stupid?) enough to get access to the source, but I don't think I would want MS on my back lol.
Dumbphones - HTC Touch Diamond - HTC Desire (A8181) - Nokia Lumia 800
OS - Windows Phone 7.5 Tango