Good point, but I'd worry about overlap, since you'd likely be touching two icons at once with an approximate touch. I suppose you could take the center point between the two touches and expand on whichever icon that lands on (with boundaries of course; it would be strange if you pinched the edges of the screen and it expanded an icon in the center).

Another thing that's bothering me about it being a two finger gesture is that the entire point of this feature idea is to be able to see things quickly at a glance. I would imagine, then, that in these scenarios most people would be holding their iPhones with one hand and only touching the screen with one thumb. This wouldn't be such a big deal on iPad, but I think it would be on iPhone. That makes me feel more strongly that some kind of one finger gesture may be a better way to do this.

Other people in this thread have pointed out some good points too about discoverability: which apps support the feature? How would the API be designed in a way that's performance and power efficient?

This is definitely fun stuff to think about :)