It's not restricted to 2-2 per se. The top and bottom halves have the same name, so you can have up to 4 in any combination. Some people run 3 of one half and 1 of another, or two of each half. You could run four of one half if you wanted to, but you wouldn't get very far if you were trying to put them into play.
But you can't have four of the top half and four of the bottom half of the same LEGEND in the same deck.