HTML Code:

<div class="row">
  <div class="col-6">
    <label>Main</label>
    <input id="main" class="form-control" placeholder="Enter some text">
  </div>
  <div class="col-6">
    <label>Mirror</label>
    <div id="mirror"></div>
  </div>
</div>

Javascript:

var main = document.getElementById('main');
var mirror = document.getElementById('mirror');

main.addEventListener('input', function(event) {
  mirror.innerText = event.target.value.split('').join('');
});

Sample: