JSFiddle - React, Tailwind, and code Playground

HTML

Source1 <input type="text" class="source" id="source1"><br/>
Source2 <input type="text" class="source" id="source2"><br/>
<br/>
Dest1 <input type="text" id="dest1">

JavaScript

var s1HTML
  var s2HTML
$('.source').keyup(function() {
  s1HTML = $('#source1').val();
  s2HTML = $('#source2').val();
$('#dest1').val(s1HTML + ' ' + s2HTML);
});