It’s already in the Documentation https://getbootstrap.com/docs/4.3/layout/grid/#responsive-classes
You can use
<div class="container">
<div class="row">
<div class="col-sm-12 col-md-6">
<div class="panel panel-blue">This is a column.</div>
</div>
<div class="col-sm-12 col-md-6">
<div class="panel panel-purple">This is a column.</div>
</div>
<div class="col">
<div class="panel panel-purple">This is a column.</div>
</div>
<div class="col-sm-12 col-md-6">
<div class="panel panel-purple">This is a column.</div>
</div>
<div class="col-sm-12 col-md-6">
<div class="panel panel-purple">This is a column.</div>
</div>
</div>
</div>