Nintendo might do something like that, but I don't think it would make sense to have a handheld unit as the supplemental unit.
If we're talking about a handheld, Nintendo would want it to be relatively inexpensive to appeal to their core market. It's also going to have to be small and enclosed. So, I'm not sure how much power they can squeeze into it. If it's as powerful as the Vita, that'd be a drop in the bucket compared to the PS4's power. Even if it was more in line with the Shield tablet, we're still talking a pretty modest boost.
I think it's more likely the supplemental device would be a straight up video card. Nintendo seems oddly concerned with the size of their console (maybe more of a Japanese market thing) and having a GPU on a separate thing would enable them to keep the actual device pretty small.
Or of course it could just be something that never makes it beyond the patent phase.







