Still space between elements (buttons) with "margin: 0"?

Just remove any spaces between DIVs:

    <div class="redline_changes">
        <a href="#">Left Button</a>
    </div><div class="redline_comments">
        <a href="#">Right Button</a>
    </div>

The two buttons have a space between them because of the return and the indentation you used to format your HTML.

Just float both containers: http://jsfiddle.net/TRVfh/19/

.redline_changes, .redline_comments{
    margin-left: 0px;
    margin-right: 0px; float: left;
}

The space is caused by the newline and indentation between the two internal divs. Removing the white space will bring the two closer.

<div class="shell" style="width: 950px; ">
        <div class="redline_changes">
            <a href="#">Left Button</a>
        </div><div class="redline_comments">
            <a href="#">Right Button</a>
        </div>
</div>