If i underatand correctly i think you are asking why not just use one pad instead of two.I understand where you are coming from as it would seem more realistic like someones head.I have seen a lot of coaches that still do only use one pad for straight punches and maybee some will mix it up and use both ways,But i think the purpose of using two pads like that teaches you to properly shift youre weight into the punches almost like exagerating the movment and maybee help you to understand the body mechanics better.although you dont want them to far apart i think most good coaches bring them to the centre to catch youre punches anyway.
Thats what i think anyway but im no expert i could be wrong.