Your idea is right but you did not calculate the border left, right of both of 2 boxes. When you calc both of them, the width of box is greater than 50%.
I also update some codes as below:
<div class="row row1">
<div class="paper">
<h1>USDT Wallet</h1>
<h3>$0.00</h3>
</div>
<div class="paper">
<h1>Convert</h1>
<button>Convert USDT to LTC*</button>
<br><br>
<button>Convert LTC to USDT*</button>
<p>*1% fee applies</p>
</div>
</div>
<div class="row row2">
<div class="paper">
<h1>USDT Wallet</h1>
<h3>$0.00</h3>
</div>
<div class="paper">
<h1>Convert</h1>
<button>Convert USDT to LTC*</button>
<br><br>
<button>Convert LTC to USDT*</button>
<p>*1% fee applies</p>
</div>
</div>
CSS:
.row{
height: 100%;
width: 49.5%;
display: inline-block;
}
.row1{
background-color: red;
}
.row2{
background-color: blue;
}