How to make one out of two columns, in bootstrap 4. So that the contents of the two columns become one?

Question

Bootstrap

How to make one out of two columns, in bootstrap 4. So that the contents of the two columns become one?

<div class="row">
        <div class="col-3 col-sm-2">
          <div class="header-elem">
            <div class="header-elem_img">
              <img src="img/header/logo.png" alt="logo.png" class="logo-header">
            </div>
            <!-- /.header-elem_img -->				
          </div>
          <!-- /.header-elem -->
        </div>
        <!-- /.col-3 -->

        <div class="col-3 col-sm-3">
          <div class="header-elem">
            <div class="header-elem_img">
              <img src="img/header/mark.png" alt="mark.png" class="mark-header">
            </div>
            <!-- /.header-elem_img -->
            <div class="header-elem_text">
              <p class="address-header">
                г. Москва,<br> Авиамоторная улица, 4А
              </p>
              <!-- /.address-header -->
            </div>
            <!-- /.header-elem_text -->
          </div>
          <!-- /.header-elem -->
        </div>
        <!-- /.col-3 -->

        <div class="col-3 col-sm-4">
          <div class="header-elem__right">
            <div class="header-elem_img">
              <img src="img/header/convert.png" alt="convert.png" class="convert-header">
            </div>
            <!-- /.header-elem_img -->
            <div class="header-elem_text">
              <p class="mail-header"><a href="mailto:[email protected]"></a>[email protected]</p>
              <!-- /.mail-header -->
            </div>
            <!-- /.header-elem_text -->
          </div>
          <!-- /.header-elem -->
        </div>
        <!-- /.col-3 -->

        <div class="col-3 col-sm-3">
          <div class="header-elem__right2">
            <div class="header-elem__right2-text">
              <p class="phone-header">[phone]</p>
              <a class="link" href="#">Заказать макетные запчасти</a>
              <!-- /.phone -->
            </div>
            <!-- /.header-elem_img -->												
            </div>
            <!-- /.header-elem_text -->
          </div>
          <!-- /.col-3 -->
        </div>
        <!-- /.row -->

Answer 1

if it’s straight from bootstrap, then:
col3 -- *COL6* -- col3
*COL6* inside:
row > col6 -- col-6
and already these col-6 can be turned into col12 and they will become one under the other