What would even be the point of such a second console? Say the first runs games at 1800 - 2160p, but what about the latter? It's more powerful than it would need to be for 1080p only, and it would still sell for a loss or at no more than $100 less with such a small difference in APU size, cooling etc. between the two.
That would be like if Microsoft released the Xbox One X but it had hardware similar to the PS4, instead of a significant difference. The 4 - 12 TF gap, on the other hand, makes far more sense cost-wise, and the is closer by analogy to the three platforms - PS4, Xbox One, Switch - that have already offered tiers based on resolution.
MS is aiming for 2160p for all games on Series X I believe, and is urging 3rd party developers to do the same. As for Series S, 7-8 tflop should be enough to aim for 1440p or a bit higher on the same games. That image could either be downscaled to 1080p for those who only have 1080p tv's, providing some super-sampling benefits, or upscaled to 4K. It would give budget oriented gamers a cheaper next-gen option that they can use while they still have a 1080p tv, which they can also use for awhile after upgrading to a 4K tv later in the gen, before they either upgrade to Series X or whatever the mid-gen refresh Xbox console will be called.