When creating a Nintex workflow, one of the actions you can add is a "to-do task". You are then able to enter multiple assignees for this task. One of the task options is "First response applies". This means that as long as one of the assignees approves, the workflow can continue without waiting for all assignees to approve.
When this workflow action kicks off, a list item will be entered for each assignee in the associated Tasks list. By default, when one of the assignees completes the task, the other list items remain in a "Not Started" state. To get around this, you want to add the "Nintex Workflow Multi Outcome Task" content type to the Tasks list. Now, when the first person completes the task, the other tasks will automatically be completed.