Liquid: loop index reference within a loop

loop-within-loop.liquid

{% for image in page.image %}
  <img src="{{ site.image-url }}/{{ page.image[forloop.index0].src }}"
    sizes="{{ site.wide-sizes }}"
    srcset="{% assign parentloop = forloop %}
    {% for srcset1440 in site.srcset1440 %}
    {{ site.image-url }}/{{ site.srcset1440[forloop.index0] }}/{{ page.image[parentloop.index0].src }} {{ site.srcset1440[forloop.index0] }}w{% if forloop.last == false %}, 
    {% endif %}{% endfor %}"
    >
{% endfor %}

Key part: {% assign parentloop = forloop %}