You get whitespace there because you have whitespace inbetween the divs. Whitespace between inline elements is interpreted as a space.
You have:
<div id="left_side">
<div id="plan">
<h1>div 1</h1>
</div>
</div>
<div id="right_side">
<div id="news">
<h1>div 2</h1>
</div>
</div>
Change for:
<div id="left_side">
<div id="plan">
<h1>div 1</h1>
</div>
</div><div id="right_side">
<div id="news">
<h1>div 2</h1>
</div>
</div>
However, this is a bad way to do what you want to do.
You should float the elements if thats what you want to do.
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