Align two divs horizontally (one on extreme left and the other on extreme right of container)

• display:inline-block will not create a float issue so there is no need to add clearfix
• you can also use overflow:hidden instead of display:inline-block

.header {
  display: inline-block; 
  width: 100%;
  border: 1px solid red;
}
.playerOne {
  float: right;
}
.playerTwo {
  float: left;
}

<div class="header">
  <div class="playerOne">
    Oli
  </div>
  <div class="playerTwo">
    Matt
  </div>
</div>

make it simple with flex

.wrapper{ display: flex; justify-content: space-between }

<div class="wrapper"><span>1</span><span>2</span></div>

The problem is that you are not targeting the proper inline-block element. :)

.header > div{
  display: inline-block;
}
.playerOne{
  float:right;
}