Item links of a single Drupal menu, in adjacent columns, using Bootstrap.

menu--yourmenu.html.twig

{# for a generic answer to this problem, see:
http://stackoverflow.com/questions/19836567/bootstrap-3-multi-column-within-a-single-ul-not-floating-properly
Note that this example has been tailored to a single level menu of plain links; modify a copy of the core 
menu.html.twig with the changes below to keep your menu as per core.
#}
{{ menus.menu_links(items, attributes, 0) }}
{% if items %}
  {%
    set menu_classes = [
      'list-unstyled row',
    ]
  %}
  <ul{{ attributes.addClass('menu', 'nav').addClass(menu_classes) }}>
  {% for item in items %}
    {%
      set item_classes = [
        item.in_active_trail ? 'active',
        'col-xs-6',
      ]
    %}
    <li{{ item.attributes.addClass(item_classes) }}>
    {{ link(item.title, item.url) }}
    </li>
  {% endfor %}
  </ul>
{% endif %}