Originally Posted by
swag
If everyone who had a carry-on plus a personal item put the personal item under the seat in front of them, then the size of the personal item wouldn't matter (as long as it fit under seat). But many do not. And I suspect the larger the personal item is, the greater the temptation to save legroom and put both bags in the bin.
It would also be better (and fairer) if underseat space was equally divided.
Some configurations leave the window/aisle space divided by a seat strut. Other configurations take up virtually all the underseat space with entertainment hardware.