Thank you Hari. Your suggestion is helped me. But small issue, Now the instances are two. the tasks are visible to both users until they complete to it. Please let me know, if user1 completed his task then the task is not visible to that user. this scenario missing. i need to achieve this. if user1 completed his task it is visible to both users until another user also complete the task.
If you need the task to be visible to all users but only one user should complete it, then you can assign the task to a group. All users who are part of the group will be able to see that task. One of the users who wish to work on it will claim it. Once the task is claimed, then it will no longer be in the group and hence others will not be able to see it. The user who claimed it can complete the task once he did what he has to do.