If you want to update multiple tasks of an ritm if one of the task's watchlist is changed then you need to query all the tasks and update....

var gr= new GlideRecord('sc_task');

gr.addQuery('request_item',current.request_item);

gr.query();

while(gr.next())

{

if(gr.watch_list!=current.watch_list)

{

gr.watch_list=current.watch_list;

gr.update();

}

}

this will update all the other tasks associated with same ritm...