I solved it with this, adding an extra input with a type hidden and handle the IDs
<script type=”text/javascript”>
var $=jQuery.noConflict();
$(document).ready(function () {
$(“#id”).keyup(function () {
var value = $(this).val();
var cadena = value;
var letra1 = cadena[0];
var letra2 = cadena[1];
var codFinal = letra1 + letra2;
$(“#id”).val(codFinal);
});
});
</script>