Putting 2 divs under each other with Bootstrap flex classes

You can see my problem on the uploaded image. Within that, the first image.

When I minimize the browser screen width these two divs won't get under each other, just when the window is fully minimized, or when i view the site on mobile.

When the window is too small, so the 2 divs can reach each other, I want them under each other.

I tried media queries, but I think I can do this with only a few Bootstrap classes, and that will be better.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.2/dist/css/bootstrap.min.css" integrity="sha384-xOolHFLEh07PJGoPkLv1IbcEPTNtaed2xpHsD9ESMhqIYd0nLMwNLD69Npy4HI+N" crossorigin="anonymous">

<section class="header_full_width py-2 px-4">
  <div class="container-fluid">
    <div class="row align-items-center justify-content-center justify-content-lg-between">
      <div>
        <a href="tel:+3630" class="text-white_disabled header_full_width_link text-white_redundant" title="Hívás"><i class="fa fa-phone mr-1 mr-md-2"></i>Kérdésed van? +36 30 123 4567</a>
      </div>
      
      <div>
        <span class="text-white_disabled header_full_width_link text-white_redundant"><i class="fa fa-envelope mr-1 mr-md-2"></i>info@emailcimem.hu</span>
        <span class="nav_separator_2 text-white mx-1">|</span>
        
        <a href="kapcsolat" class="text-white_disabled header_full_width_link text-white_redundant" title="Elérhetőségeink"><i class="fa fa-phone-square mr-1 mr-md-2"></i>Kapcsolat</a>
      </div>
    </div>
  </div>
</section>

<!-- end snippet -->

You can use bootstrap's row and col classes.
As we can already see, in your code example above. You have used row but not col for the children div's

A bootstrap row is considered to have 12 units. So for both the div's you can give class of col-12 col-lg-6. Bootstrap takes a mobile first approach so this means, the div's will take full width of the row till the screen size reaches lg (992px), then the div's will occupy half each i.e. 6 units each.

"When the window is too small, so the 2 div's can reach each other, I want them under each other."

To answer this - you can choose when you want your div's to share the row. Just replace lg with the appropriate breakpoint by refrencing the image below.

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.2/dist/css/bootstrap.min.css" integrity="sha384-xOolHFLEh07PJGoPkLv1IbcEPTNtaed2xpHsD9ESMhqIYd0nLMwNLD69Npy4HI+N" crossorigin="anonymous">

    <section class="header_full_width py-2 px-4">
      <div class="container-fluid">
        <div class="row align-items-center justify-content-center justify-content-lg-between">
          <div class='col-12 col-lg-6'>
            <a href="tel:+3630" class="text-white_disabled header_full_width_link text-white_redundant" title="Hívás"><i class="fa fa-phone mr-1 mr-md-2"></i>Kérdésed van? +36 30 123 4567</a>
          </div>
          
          <div class='col-12 col-lg-6'>
            <span class="text-white_disabled header_full_width_link text-white_redundant"><i class="fa fa-envelope mr-1 mr-md-2"></i>info@emailcimem.hu</span>
            <span class="nav_separator_2 text-white mx-1">|</span>
            
            <a href="kapcsolat" class="text-white_disabled header_full_width_link text-white_redundant" title="Elérhetőségeink"><i class="fa fa-phone-square mr-1 mr-md-2"></i>Kapcsolat</a>
          </div>
        </div>
      </div>
    </section>