JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="" id="voornaam"
value="" class="full"/>
    <input type="text" name="" id="achternaam" value="" class="full"/>
<span class="data-name">Dhr. name lastname</span>

JavaScript

$('#voornaam').on('keyup', function () {
    var name = this.value;
    var lastname = $('#achternaam').val();
    $('.data-name').text(name + " " + lastname);
});
$('#achternaam').on('keyup', function () {
    var lastname = this.value;
    var name = $('#voornaam').val();
    $('.data-name').text(name + " " + lastname);
});