How do I add 2 blocks side-by-side in Bootstrap theme's "Content" region?
You can extend the page twig like this.

Make a copy of page.html.twig in your theme template folder and put a twig block around the content:

page.html.twig

...
{% block content %}

  {{ page.content }}

{% endblock %}
...


Then create a new file, and name it so that it will target the specific page, for example the view "myview":

page--view--myview.html.twig

{% extends "page.html.twig" %}

{% block content %}

  {{ page.content }}

  &lt;div class=".."&gt;
    {{ page.subleft }}
  &lt;/div&gt;

  &lt;div class=".."&gt;
    {{ page.subright }}
  &lt;/div&gt;

{% endblock %}


This new file extends the page twig and contains only a new version of the the twig block "content". You can define multiple twig blocks in one file and replace them.