It likes this one.

trial.js

<script type="text/javascript">
 jQuery(document).ready(function($) {
 	jQuery('#Submit').click(function() {
 	 var splitName = jQuery('#Contact0FirstName').val();
 	 var names = splitName.split(' ');
 	 var firstName = names.splice(0,1);
 	 var lastName = names.join(' ');
 	 jQuery('#Contact0FirstName').val(firstName);
 	 jQuery('#Contact0LastName').val(lastName);
 	 jQuery('form').submit();
 	 return true;
 	});
 });
</script>

Thanks. Giving it a try.