Hi Eduardo,
Rather than 'killing' the operator I would suggest that you could easily reply to the task (using the REST API) once you have a reply from either one of the Tasks.
It would disappear from their task list soon after the process is completed, if you really wanted to hide the task immediately, you could also transfer the task to a 'fake' user, and then mark it as completed.
I believe that would make the task disappear from their task list immediately.
For example, you start up two user tasks, one has Task ID 1153871, and the other Task ID 1153872.
If the first user completed his task (# 1153871), you could then use the CA PA REST API to send a ReplyTask, or a TransferTask for Task ID 1153872.
That task would then be marked as completed/cancelled and would disappear from the task list.
Using the REST sample from the Web Services Reference Guide, here is a little example of not approving TaskID 1153872 and marking is as Status = "Was completed by User 1"
<ReplyTaskRequest xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:type="p1:ReplyTaskRequest"
xmlns:p1="http://ns.ca.com/2011/09/pam-ops" >
<TaskID>1153872</TaskID>
<IsApproved>false</IsApproved>
<Arguments>
<Argument>
<Name>Status</Name>
<Value>Was completed by User 1</Value>
</Argument>
</Arguments>
</ReplyTaskRequest>
Hope this helps you!
Ian