You just have to assign the field values:

// plain JavaScript

var first = document.getElementById('firstFieldId'),

    second = document.getElementById('secondFieldId');



second.value = first.value;



// Using jQuery

$('#secondFieldId').val($('#firstFieldId').val());

And if you want to update the content of the second field live, you could use the change or keyup events to update the second field:

first.onkeyup = function () { // or first.onchange

  second.value = first.value;

};

With jQuery:

$('#firstFieldId').keyup(function () {

  $('#secondFieldId').val(this.value);

});

Check a simple example here.