openproject/app/views/settings/_backlogs_settings.rhtml

    <%= javascript_include_tag 'jquery-1.4.2.min.js', :plugin => 'redmine_backlogs' %>

    <script type="text/javascript">
        var $j = jQuery.noConflict();

        function enable_apply()
        {
            tasktracker = $j("#settings_task_tracker").val();
            submit = $j('input[type=submit]')


            disable = false;
            log = tasktracker + '::'
            $j("#settings_story_trackers :selected").each(function(i, selected){
                disable = disable || ($j(selected).attr('value') == tasktracker);
            });
            if (disable) {
                submit.attr('disabled', 'disabled');
            } else {
                submit.removeAttr('disabled');
            }
        }

        $j(document).ready(function() {
            $j("#settings_task_tracker").change(function() { enable_apply(); });
            $j("#settings_story_trackers").change(function() { enable_apply(); });

            enable_apply();
        });
    </script>

    <p>
        <%= content_tag(:label, l(:backlogs_story_tracker)) %>
        <%= select_tag("settings[story_trackers]", options_from_collection_for_select(Tracker.all, :id, :name, Story.trackers), :multiple => true) %>
    </p>
    <p>
        <%= content_tag(:label, l(:backlogs_task_tracker)) %>
        <%= select_tag("settings[task_tracker]", options_from_collection_for_select(Tracker.all, :id, :name, Task.tracker)) %>
    </p>