Yeah, I think a dual APU is the only spec bump possible, outside of a small clock numb, but extremely improbable.
One thing sweetvar mentioned in his leak last year was that AMD employees referred to the Xbox as like a super computer. Perhaps that was a reference to scalability, if designed from the beginning perhaps Durango has provisions that would allow this type of scaling with mutilple APUs. Perhaps with the rumors that the games are running in a VM and no direct hardware access, it's possible to make the system look like one giant APU to the developer. Down the road, the two APUs can be merged into one after a node shrink or two.
I think the biggest hurdle to getting something like this working would be the independent pools of ESRAM, ideally you would want that memory to be shared.
Sorry just some crazy thinking out loud, I miss the "predict next-gen hardware" thread.