In the iOS Human Interface Guidelines, Apple suggests that your touch targets have a minimum size of 44x44 points. (https://developer.apple.com/library/ios/documentation/userex...)
If you look at the icon size list at https://developer.apple.com/library/ios/documentation/userex..., you'll see that iPhone 5 icons are only 60x60 points (divide the pixel sizes by two to get the point size). I imagine that 60x60 is simply too small for a two finger gesture to work well.
Maybe this would work better with a one finger touch and drag gesture where the drag direction dictates how the icon will grow into a widget (but with icons on the top and bottom rows only able to grow in one direction). It would need to be a quick gesture, though, since tap and hold triggers that app reorganization mode of Springboard.