While the iPhone 4 screen's aspect ratio is 1.5:1, but the iPhone itself is 115.2 mm x 58.66 mm (approx 1:2). So, the same reasoning should apply to two finger use horizontally.

But many people use their phone horizontally with both hands.

See: text messagers, IRC users, ssh session runnners, angry birds players

And I doubt that you'll have any trouble using the iPhone 5 with two hands.

Hey look! It's a PS Vita without the backside touch interface or media card reader! ;-)

Depends on the phone.