{% comment %} 
Generate list of mixed articles from all listed blogs, sorted by publish date.
Add the blog handles you want to include in the output, as many as you need.
{% endcomment %}
{% assign all_blogs = "blogHandle1,blogHandle2,blogHandle3" | split: "," %}
{% assign article_list = "" | split: "," %}
{% assign delimiter = "___" %}
{% for blog_handle in all_blogs %}
  {%- for item in blogs[blog_handle].articles -%}
    {% assign identifier = item.published_at | date: "%Y-%m-%d-%H-%M-%S" | append: delimiter | append: item.handle | split: " " %}
    {% assign article_list = article_list | concat: identifier %}
  {%- endfor -%}
{%- endfor -%}
{% assign article_list = article_list | sort_natural | reverse %}

{% comment %} 
Create markup for posts 
{% endcomment %}
<!-- any container start markup -->
{% for identifier in article_list limit:20 %}
  {% assign identifierParts = identifier | split: delimiter %}
  {% comment %} Get post from global articles object, using the "handle"/URI {% endcomment %}
  {% assign item = articles[identifierParts[1]] %}
  <!-- your per-post markup with variables like item.title, item.url, item.image, etcetera -->
  <!-- {{ identifier }} -->
{%- endfor -%}
<!-- any container end markup -->