A solution using bootstrap classes
.rhs-div {
border: 1px solid red;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
<!-- Stack the columns on mobile by making one full-width and the other half-width -->
<!-- Columns start at 50% wide on mobile and bump up to 33.3% wide on desktop -->
<div class="container d-md-flex">
<div class="row mx-0">
<div class="col-md-12" style="border:1px solid red">
<strong>LHS DIV</strong> This is a div which can have a dynamic Lorem ipsum dolor sit amet consectetur adipisicing elit. Labore esse quae quis eaque ab? Magnam tenetur id ducimus laudantium, optio quisquam eos ut dolorum sunt iure deserunt deleniti
delectus voluptatibus.
</div>
</div>
<div class="row mx-0">
<div class="col-md-9 d-md-none">
</div>
<div class="col-md-12 rhs-div "><strong>RHS div</strong></div>
</div>
</div>
Another one modifying only the CSS:
.rhs-div {
border: 1px solid red;
}
@media (min-width:767px) {
.container {
display: flex;
}
.container .row{
margin:0;
}
.container .row .col-md-9,
.container .row .col-md-3{
width:100%!important;
max-width:100%!important;
flex: 0 0 100%;
}
.container .row:last-child .col-md-9 {
display:none;
}
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
<!-- Stack the columns on mobile by making one full-width and the other half-width -->
<!-- Columns start at 50% wide on mobile and bump up to 33.3% wide on desktop -->
<div class="container">
<div class="row">
<div class="col-md-9" style="border:1px solid red">
<strong>LHS DIV</strong> This is a div which can have a dynamic Lorem ipsum dolor sit amet consectetur adipisicing elit. Labore esse quae quis eaque ab? Magnam tenetur id ducimus laudantium, optio quisquam eos ut dolorum sunt iure deserunt deleniti
delectus voluptatibus.
</div>
</div>
<div class="row">
<div class="col-md-9">
</div>
<div class="col-md-3 rhs-div"><strong>RHS div</strong></div>
</div>
</div>