Drupal Bootstrap - Regions Placement
With Bootstrap you can have two regions side by side wrapping them with a row and adding a col-size-number class to them, like this:

&lt;div class="row"&gt;
  &lt;div class="region region-name-left col-md-6"&gt;
    ...
  &lt;/div&gt;
  &lt;div class="region region-name-right col-md-6"&gt;
    ...
  &lt;/div&gt;
&lt;/div&gt;


You have to do that in the page.html.twig file.