Select second 2nd child div from within the parents div

You need to remove inline style from the second div tag

<div style="border-top: 1px solid #e5e5e5; width: 100%; line-height: 22px;">

It is overriding your css style.

For whatever reason if you want to keep the style, use !important although I highly DO NOT recommend this approach because it is very very bad practice but that's an option (however bad).

    .manufacturer_box div:nth-child(2) {
            border-top: 5px solid #e0e0e0 !important; 
     }

The correct selector is use a child selector ">" to make sure that only the second child div, of the parent div, will be selected. If you don't use > and only use:

.manufacturer_box div:nth-child(2){
    background: #FF0000;
    height: 200px; /* Manually set */
 }

This code is not specific enough, in the provided html will select "hello 1" and "hello 2" div's.

So the correct selector is:

CSS

.manufacturer_box > div:nth-child(2){
    background: #FF0000;
    height: 200px; /* Manually set */
 }

This will select only "hello 2" div.

HTML

<div class="manufacturer_box">
    <div class="manufacturer_title">
        <h1>Title 1</h1>
        <div>
            <span style="color: #999999; font-size: medium;">hello 1 <br> 
            <span style="color: #333333;"></span> </span>
        </div>
    </div>
    <div>
        <span style="color: #999999; font-size: medium;">hello 2 <br> 
        <span style="color: #333333;"></span> </span>
    </div>
</div>

Here's the snippet, remove the ">" to test that it selects multiple divs, and with ">" selects only the correct one.

.manufacturer_box > div:nth-child(2){
    background: #FF0000;
    height: 200px; /* Manually set */
 }

<div class="manufacturer_box">
    <div class="manufacturer_title">
        <h1>Title 1</h1>
        <div>
            <span style="color: #999999; font-size: medium;">hello 1 <br> 
            <span style="color: #333333;"></span> </span>
        </div>
    </div>
    <div>
        <span style="color: #999999; font-size: medium;">hello 2 <br> 
        <span style="color: #333333;"></span> </span>
    </div>
</div>