Question

How do I handle having two separate jQuery libraries? I need both of them, but including both results in a conflict, and only one’s functions will work.

Answer 1

It depends. You can actually have both of them active in the page at the same time, but only one of them can have the $ symbol, and only one of them can have the jQuery symbol. But technically, you can have more than one on a page:

<script src="jquery-1.5.2.js"></script>
<script>var $j15 = jQuery.noConflict(true);</script>
<script src="jquery-1.7.1.js"></script>
<script>
    // Here, $ = jQuery = jQuery 1.7.1
    // $j15 = jQuery 1.5.2
</script>

Note that a well-authored jQuery plug-in will work even in that environment, e.g.:

<script src="jquery-1.5.2.js"></script>
<script src="plugin-requiring-1.5.2.js"></script>
<script>var $j15 = jQuery.noConflict(true);</script>
<script src="jquery-1.7.1.js"></script>
<script>
    // Here, $ = jQuery = jQuery 1.7.1
    // $j15 = jQuery 1.5.2
    $j15("selector").pluginRequiring152(...);
</script>

…because a well-authored plug-in looks like this:

(function($) {
    $.fn.pluginRequiring152 = ...;
})(jQuery);

…and so will use whatever is jQuery at the point you include it, without breaking if you later fire off noConflict(true).

Is it a good idea? Almost certainly not. Best to resolve whatever the issues are that make you need more than one version of jQuery, and only include the latest you’ve tested with.