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

        
css
                 #c1{
    display: inline-block;
    width: 49.8%;
    background-color: red;
}

#c2{
    display: inline-block;
    width: 49.8%;
    background-color: blue;
}

@media  screen and (max-width: 740px){
    #c1 , #c2{
        width: 100%;
    }
}
 
            

            
js