Could I get more information about the max weight it holds? In the desciption it says: up to 7 lbs (3,175 gm). I assume it is the total weight it can handle, despite the 2- or 3- tier variant? Let say i have 1 keyboard of 4 kg and one of 2 kg , total of 6kg (13 lbs). Will that be a problem? Or does it hold up to 7 lbs per keyboard?