I was looking for a way to do this, turns out twig has exactly this feature! http://twig.sensiolabs.org/doc/tags/embed.html

foo.html.twig

{# hypothetic twig tag "with" #}
{% with 'panel.html.twig' %}
  {% block panel_title %}This is the panel title{% endblock %}
  
  {% block panel_body%}
    <p>This is the panel body</p>
  {% endblock %}
{% endwith %}


{% with 'panel.html.twig' %}
  {% block panel_title %}This is another panel title{% endblock %}
  
  {% block panel_body%}
    <p>And the body</p>
  {% endblock %}
{% endwith %}

panel.html.twig

<div class="panel panel-default">
  <div class="panel-heading">
    <h3 class="panel-title">{% block panel_title %}{% endblock %}</h3>
  </div>
  <div class="panel-body">
    {% block panel_body %}{% endblock %}
  </div>
</div>