html
                 <div id="main">
    <div id="c1">1</div>
    <div id="c2">2</div>
</div>
 
            

        
css
                 #main {
    display: flex;
}

#c1 {
    background-color: blue;
    flex: 1;
    height: 100px;
}

#c2 {
    background-color: red;
    flex: 1;
    height: 100px;
}

@media  screen and (max-width: 740px) {
  #main {
    display: block;
  }
}
 
            

            
js