<?php
/**
 * The template used for displaying page content
 *
 */
?>

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

	<header class="entry-header">
		<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
	</header><!-- .entry-header -->

	<div class="entry-content">
		<?php the_content(); ?>
		<?php
			wp_link_pages( array(
				'before'      => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentyfifteen' ) . '</span>',
				'after'       => '</div>',
				'link_before' => '<span>',
				'link_after'  => '</span>',
				'pagelink'    => '<span class="screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>%',
				'separator'   => '<span class="screen-reader-text">, </span>',
			) );
		?>
	</div><!-- .entry-content -->

</article><!-- #post-## -->

<?php
/**
 * The template used for displaying page content
 *
 */
?>

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

	<header class="entry-header">
		<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
	</header><!-- .entry-header -->
	<div class="entry-content-container">
		<div class="entry-content">
			<?php the_content(); ?>
			<?php
				wp_link_pages( array(
					'before'      => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentyfifteen' ) . '</span>',
					'after'       => '</div>',
					'link_before' => '<span>',
					'link_after'  => '</span>',
					'pagelink'    => '<span class="screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>%',
					'separator'   => '<span class="screen-reader-text">, </span>',
				) );
			?>
		</div><!-- .entry-content -->
		<nav class="entry-nav">
			<ul class="page-menu">
				<!-- insert -->
			</ul>
		</nav>
	</div><!-- .entry-content-container -->
</article><!-- #post-## -->

Add container div for two-column layout
Add container div for two-column layout
