Can I use theme options to change two things at once?

Nothing prevents you from retrieving your option in several places. You can do several get_option() calls or create global variable and assign your value to it once.

It is hard to come up with specific example without knowing what exactly you store in option and which classes you want to generate for your markup.

Update

One of the ways to get it done. Depending on how you organize code you might need to declare variables as global :

$options = get_option('sample_theme_options');
$sidebar_width = $options['sidebar_width']; // let's say we have stored integer '2' here
$column_width = 8 - $sidebar_width; // total width minus sidebar

// and then in template
echo "<div class="main-{$column_width}col"> main </div>";
echo "<div class="sidebar-{$sidebar_width}col"> sidebar </div>";