If you want to include both the js files you can..
<!-- load jQuery 1_8_3 -->
<script src="../assets/js/jquery-1.8.3.min.js"></script>
<script type="text/javascript">
var jQuery_1_8_3 = $.noConflict(true);
</script>
<!-- load jQuery 1.4.2 -->
<script type="text/javascript" src="jquery/jquery-1.4.2.js"></script>
<script type="text/javascript">
var jQuery_1_4_2= $.noConflict(true);
</script>
Its better to avoid multiple versions in the page..and its better to use appropriate jquery-UI Version with the jquery version
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