Can you imagine the chaos of having two (or more) "cabins" in the same space. Unless the cabin service is going to be nearly identical (like NZ had on their 747 upper deck, front rows J, rear Y+, but bascially the same catering) then I can only see this getting very complicated. It would be terribly easy for someone to sneak into a better seat en route, as well as the aforementioned awkwardness of having people getting A seats & A service, whilst surrounded by people getting B seat and B service.

Since airlines have broken away from the traditional (e.g. 2x3x2 rows) of seating and gone for more orginal designs with inventive use of space, facing random directions, I don't see a particularly strong need to mix cabins for marginally better space utilisation, with all the associated dilution of the more expensive product.