A 2nd CPU, with different architecture too, what's worse, for the OS just doesn't make sense: POWER is a very scalable architecture and simply adding another core to the CPU, if necessary, would cost a little less for the total cost of the components and a lot less for the overall cost of SW and HW architecture design, while delivering far better performances, as the OS reserved core would communicate far faster with the other cores, and without the need of a translation layer.
Different architecture for the OS CPUs are used, afaik, in some massively parallel supercomputers, but they are designed to work at their best on applications quite different from videogames, and they require to write programs specifically for them, this would make ports to and from them more difficult and expensive. An example: http://en.wikipedia.org/wiki/IBM_Roadrunner
Edit: PS I just read what some of you wrote about the ARM CPU used in the Wii, that's quite a different thing, it's used as a coprocessor or a HW accelerator, programs running in the CPU just feed to it some calculations it's best at, just like they do the same thing with the GPU. So a ARM CPU can be used in Wii U not to run the OS, but simply for the same purposes it was used for in the Wii.
