Edit in JSFiddle

$(document).ready(function() {
    $('#txtFirst').keyup(function(e) {
        var txtVal = $(this).val();
        $('#txtSecond').val(txtVal);
    });
    
    $('#txtSecond').keyup(function(e) {
        var txtVal = $(this).val();
        $('#txtFirst').val(txtVal);
    });
});

<b>Value 1 </b>: <input id="txtFirst" type="textbox" />
<br/><br/>
<b>Value 2 </b>: <input id="txtSecond" type="textbox" />
<br/>