you can't from jBPM directly. If you assign to a pool of users, ONE of those has to act on it.
What you can do is develop a web interface so that if one of the users acts on it, you do not end the task, but remove specific users from the pool and reassign back to the pool (by making the actor 'null'). Only if a (or b, but what he/she is you do not describe) fills in the variables you close the task
this does require a custom webapplication though
Thanks for your reply
Can this be added as an enhancement or this is a custom requirement?
To me it feels it is a generalised requirement