You need to start Row class after every 2N+1 post.
And end the Row class after every 2N+2 Post.
Try this one
<?php $counter++;
if ($counter % 2 == 1) : ?>
<?php echo '<div class="post-wrapper">'; ?>
<?php endif; ?>
<?php get_template_part( 'templates/blog/blog-2-cols', get_post_format() );?>
<?php $counter++;
if ($counter % 2 == 0) : ?>
<?php echo '</div">'; ?>
<?php endif; ?>