The div is a block-level element, i.e it occupies the whole of the page width and the siblings are place one below the other irrespective of their width.

<div>
    <p>This is DIV 1</p>
</div>
<div>
    <p>This is DIV 2</p>
</div>

The output of the following code will be

We can make them in-line by adding a float css property to the div.

HTML:

<div class="outer-div">
    <div class="inner-div1">
        <p>This is DIV 1</p>
    </div>
    <div class="inner-div2">
        <p>This is DIV 2</p>
    </div>
</div>

CSS

.inner-div1 {
    width: 50%;
    margin-right:0px;
    float:left;
    background : #337ab7;
    padding:50px 0px;
}
 
.inner-div2 {
    width: 50%;
    margin-right:0px;
    float:left;
    background : #dd2c00;
    padding:50px 0px;
}
 
p {
    text-align:center;
}

Codepen Link