I don’t have time for a detailed reply but some ideas:
1) Use arrays in the front end. Make your input ID’s like add_task_assigned[]
and add_task_complete[]
– these appear as arrays in the back end. It’s not great practice, but it works.
2) Use an array in the backend. Store the meta as
array(
[0] => array( 'assigned' => true, 'completed' => true ),
[1] => array( 'assigned' => true, 'completed' => false )
)
3) As we’re not quite sure how you’ve structured the HTML, the serialize method of jQuery UI sortable may also be handy: http://api.jqueryui.com/sortable/#method-serialize