Currently exists only the undocumended(?) plural form [lists_to_confirm] which is replaced by the new list one wants to subscribe to as well as all already subscribed lists (separated by comma). This is in violation to the name of the shortcode.
But even if [lists_to_confirm] would be replaced by what it name suggests it would be stupid:
Assume the following example: Even if I subscribe to two lists one after another I would never expect to get first “list 1” mentioned in a mail and in the second mail (before I confirm) “list 1, list 2”.