// Create two column layout
function [site_name]_archive_post_class( $classes ) {
	global $wp_query;
	if( ! $wp_query->is_main_query() )
		return $classes;

	$classes[] = 'forty-five';
	if( 0 == $wp_query->current_post || 0 == $wp_query->current_post % 2 )
		$classes[] = 'first';
	return $classes;
}
add_filter( 'post_class', '[site_name]_archive_post_class' );

//CSS
.forty-five {
		float: left;
		width: 45%;
		@media (max-width: 480px) {
			width: 100%;
		}
		&.first {
			margin-right: 10%;
		}
	}