If you want the items to be grouped inside a row, you can use this code instead
{% for content in contents %}
{{ '<div class="row">' if loop.index % 2 != 0 }}
<div class="post-item {% cycle 'width-one-third','width-two-thirds', 'width-two-thirds','width-one-third' %}">
...
</div>
{{ '</div>' if loop.index % 2 == 0 }}
{% endfor %}