pass block into embed and forward to another embed?
If partial_with_form.twig isn’t making any of its own customizations to form.twig, then I’d just include the form.twig template using an {% include %} tag, and wrap that whole thing with a {% block %}, so if a template wants to include the partial and make changes to the form, they can just redefine the form themselves.

partial_with_form.twig:

{% block form %}
  {% include 'form' %}
{% endblock %}


Other template:

{% embed `partial_with_form` %}
  {% block form %}
    {% embed 'form' %}
      {% block form_extension %}
        &lt;!-- ... --&gt;
      {% endblock %}
    {% endembed %}
  {% endblock %}
{% endembed %}