8-4-4 grids to 12-4

Hello,

I've been trying to figure it out how can I change the grids or the layout dynamically. I would like the layout to be like this,

<div id="main-content" class="grid_16">
  <div id="content" class="grid_8">

  </div>

  <div id="sidebar-first" class="grid_4">

  </div> 

  <div id="sidebar-second" class="grid_4">

  </div>
</div>

So, basicly the layout is the same that it's in the demo here, http://ninesixty.fkdemos.com/tests/8-4-4 . But I've been trying to figure out is it possible, if there's no sidebar-first in the layout can I change it somehow dynamically to 12-4. Like below,

<div id="main-content" class="grid_16">
  <div id="content" class="grid_12">

  </div>

  <div id="sidebar-second" class="grid_4">

  </div>
</div>

Hopefully, someone can help me with my problem. It's been abit frustrating to try to figure this one out all by myself.

Yours sincerely,

Jussi Jokinen