Yeah... that would require a completely custom motherboard (or having two motherboards). I don't think there's any hardware right now capable of handling having two CPUs of completely different architectures in the same machine. If you want them to share memory and other peripherals, we're talking a completely new kind of north/southbridge architecture that would have to be developed just for this device. Just waiting for Intel to make more power efficient CPUs will probably faster than starting this kind of project from scratch.
Then you'd need to have two operating systems. Windows RT can't run on Intel, and Windows 8 can't run on ARM, so you'd have to dual-boot. This takes twice as much storage space, and you require a reboot every time you want to switch; not exactly user-friendly. And even if you can somehow sync which apps are installed and start screen layout etc. between the two, you'd still need to install them twice (since ARM and Intel apps can use different binaries), again wasting more disk space.
It's a nice idea, but actually doing it in a practical fashion is pretty much impossible without a complete and utter redesign of both hardware and Windows itself.