Hello  @Sumit 

Create a datalabel in ur smartform , Make sure to have literal property checked for this data label .

Note: In my scenario , my first textbox name = a

my second textbox name = b

Add expression to this datalabel , which has the following 

<script type="text/javascript">
$('[name="a"]').keyup(function(event){
$('[name="b"]').removeClass("disabled") ;
$('[name="b"]').removeAttr("disabled");
});
</script>

on change on textbox named "a" , it will enable the second textbox "b"