You can specify many actor or group ids separated by comma and they will all be added to potential owners. After that, onceof then can claim, work and complete the task. Does it help?
When I specify multiple actorid to single human task there is only one actor can be a potential owner.
If one of the actor claim the task he becomes the potential owner and task disappears from the other actor also. I want that both the actor can assign the task and when all of the actors complete the task it should go to other task.
Looks like you are more interested in multi instance characteristic of a task. Meaning engine will create task instance for each user/actor that is given as a collection. So all actors will have task to work on and if you like to constrain the flow to continue you can put a gateway after task.
Take a look at following example.
Yes, you can also use the sub tasking feature.. I've created that like 2 years ago so I don't know if it is still working but you can check that and test it.