Are the "two second-tier bodies" you refer to Sony and Sigma, or what?
No. First tier = Canon 1D X, Nikon D4. Second tier = Canon 5DIII/X, Nikon D800.
Thanks Neuro. I guess I didn't make myself clear.
I think I would go with those that say that at the 1DX and D4 level, there is a very narrow customer base and their needs are consistent and well understood by both companies, so the bodies tend to converge.
On the other hand, I would also agree with the idea that for the 5D and D800 both companies were trying to address the complaints of their customers.
In my mind, this confirms something I've thought about. We tend to think Canon and Nikon are in some sort of death battle to steal each other's customers. But, I suspect they are far more interested (at this level at least) in keeping their base happy and solidifying that base of customers. Old axiom that it is always cheaper and more profitable to retain an existing customer and "upsell" that customer, than it is to capture a new customer.
I expect both companies will watch the sales of their respective products and if the sales are disappointing we may see some adjustments, but I don't think we can automatically assume that Canon will introduce a mega-pixel killer just because Nikon has one, nor do I expect Nikon to automatically introduce a lower megapixel model just because Canon has one.