@media only screen and (max-width: 400px) {

#content_menu{
    position: relative;
    height: 1500px;
}

#seccion_uno{
    position: static;
    height: 750px;
    width:400px; 
}

#seccion_dos{
    height: 750px;
    position: static;    
    width:400px; 
}

}