HI Xiaojun,
In this case both tasks wil be created, That's fine. The problem is that your main() method will die along with the sessions right?
The sessions will not be notified about the task completion, for that reason your process will not continue to the next steps.
For this reason we usually configure the session with a persistence mechanism that allows us to restore the status of the session from a different thread and notify it when a task was completed. You should also need to consider using CommandBasedWSHumanTaskHandler instead of WSHumanTaskHandler if you are planing to use persistence.
By the way who is starting the Human Task Server? are you getting some error?
Cheers