Think of a device like an Asus Transformer. In the tablet part is an Atom CPU, and when its in "tablet mode" it uses Metro.
Then you put it in the dock, which has an i5 built in, and it goes into "normal Windows mode."
Or just wait for 7W Haswell parts that are out in 5 months. The solution you are proposing would require Intel to dedicate resources to it, take 1-2 years to implement, and ultimately would suck. Plus, you'd end up with sheer duplicates of a lot of system resources, leading to a machine that would be $200 more expensive that it really needs to be.
Again, it would suck. 7W Haswell is just around the corner and next-gen Atom should be here by the end of the year.