I don't see how anyone would benefit from such a system.
Developers and publishers would only suffer an increased budget and longer schedules for their titles.
Higher resolution assets take more time; not everyone paints textures at twice the resolution, and even they don't paint textures that'd actually work at that resolution. Besides, keeping 1K textures at 2K would quadruple the memory requirements, and not just double it.
Significantly more Q&A would have to be done because the entire matrix is expanded. And they'd need to basically build and maintain two versions of their game, as moving to 60fps isn't just about rendering more frames, game logic and physics and AI and I/O and network and everything else would have to be modified as well. But even a simple resolution icrease would mean lots of extra work.
All these would cost a lot of money, but they wouldn't really sell that much more of the same game and it wouldn't really provide a competitive edge either. Most casual console gamers I know still think that COD Black Ops is actually rendering at 1080p...
Console vendors wouldn't really benefit either. You can't count on the majority of your user base to upgrade, as most people want cheap consoles, so the economics of the expansion set would not allow for a good price/performance ratio. Either it'd have to be hideously expensive (and also increase the cost of the base system even if it won't get upgraded) or it'd not have enough computing power to make a difference. It'd also take away engineering resources from more important projects, it'd make the replacement generation of hw somewhat harder to differentiate, it'd complicate hw revisions, it'd also mess with the initial design in terms of power consumption and cooling.
And most importantly, they wouldn't make any more money with such an upgrade either. If the kit is sold at a profit it'd be too expensive; if it'd be a loss leader it'd have to move significantly more games which it couldn't on its own, it'd also cost a lot of R&D money.
And finally, how would the average customer benefit from this? Noone likes to be forced to upgrade and as I've said most people wouldn't even notice small scale differences.
Sure there are like 5-10% of the market of the PS3/X360 who'd maybe even go out and buy this thing but they still wouldn't get enough games that'd be significantly upgraded, for all the reasons mentioned above.
Stuff like Move/Kinect or online features are far, far better investments for everyone in every possible way, whereas a hw upgrade doesn't make any sense at all...
Oh and connecting multiple consoles is silly as well, it has the same drawbacks on the dev/publisher/manufacturer side. Develop two different versions, market it somehow, modify the base design kust for at most 5-10% of your user base, and so on...