Boarding without allocated seat numbers is quicker because everyone attempts to board first to get the best seats and you do not get people hanging around in bars till the last minute.
Boarding through 2 doors should be quicker providing the passengers obey the ground staff and board through the door they are directed to. My experience is as crew you get quite a few who ignore the instructions and board through the forward door regardless of where they are sitting. Even some in the last row will out of habit board through the forward door. This of course defeats the object of 2 door boarding as you get a pile-up in the aisle.
Unless a member of ground staff checks the seat numbers and insists they board though the door closest to their seat it will not work. People have a habit of always going for the forward door for some reason.