footer.php

<?php
/**
 * The template for displaying the footer
 *
 * Contains the closing of the "site-content" div and all content after.
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */
?>

	</div><!-- .site-content -->

	<footer id="colophon" class="site-footer" role="contentinfo">
		<div class="site-info">
			<?php
				/**
				 * Fires before the Twenty Fifteen footer text for footer customization.
				 *
				 * @since Twenty Fifteen 1.0
				 */
				do_action( 'twentyfifteen_credits' );
			?>
			<?php if ( is_active_sidebar( 'footer-sidebar-1' ) ) : ?>
				<div id="footer-sidebar" class="footer-sidebar footer-sidebar-left widget-area" role="complementary">
					<?php dynamic_sidebar( 'footer-sidebar-1' ); ?>
				</div><!-- #primary-sidebar -->
			<?php endif; ?>
			<?php if ( is_active_sidebar( 'footer-sidebar-2' ) ) : ?>
				<div id="footer-sidebar" class="footer-sidebar footer-sidebar-right widget-area" role="complementary">
					<?php dynamic_sidebar( 'footer-sidebar-2' ); ?>
				</div><!-- #primary-sidebar -->
			<?php endif; ?>
		</div><!-- .site-info -->
	</footer><!-- .site-footer -->

</div><!-- .site -->

<?php wp_footer(); ?>

</body>
</html>

functions.php

<?php

function twentyfifteen_parent_enqueue_styles() {
	wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}
add_action( 'wp_enqueue_scripts', 'twentyfifteen_parent_enqueue_styles' );

function twentyfifteen_footer_sidebar_widgets_init() {
	register_sidebar( array(
		'name'          => __( 'Footer Sidebar 1', 'twentyfifteen-footer-sidebar' ),
		'id'            => 'footer-sidebar-1',
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h2 class="widget-title">',
		'after_title'   => '</h2>',
	) );
	register_sidebar( array(
		'name'          => __( 'Footer Sidebar 2', 'twentyfifteen-footer-sidebar' ),
		'id'            => 'footer-sidebar-2',
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h2 class="widget-title">',
		'after_title'   => '</h2>',
	) );
}
add_action( 'widgets_init', 'twentyfifteen_footer_sidebar_widgets_init', 11 );

style.css

/**
Theme Name: TwentyFifteen with Footer Sidebar
Template: twentyfifteen
*/
.site-footer .site-info:after {
	content: " ";
	display: table;
	clear: both;
	margin-bottom: 20px;
}
.site-footer .footer-sidebar {
	margin: 0;
	max-width: 300px;
	width: 47%;
}
.site-footer .footer-sidebar-left {
	float: left;
}
.site-footer .footer-sidebar-right {
	float: right;
}
.site-footer .footer-sidebar aside {
	margin: 0;
	padding: 0;
}
/**
 * 16.1 Mobile Large 620px
 */

@media screen and (max-width: 620px) {
	.site-footer .footer-sidebar {
		float: none;
		max-width: 100%;
		width: 100%;
		margin-bottom: 20px;
	}
}