Normally using 2 items of the same type does not stack. On the same commander at least. So you want to give each of your siege commanders a horn and an axe. But not two horns. (iirc of course).
If you are really serious about taking down those walls, you could also use some
http://dom3.servegame.com/wiki/Siege_Golem But a bit expensive perhaps at 20 gems in the base game.