Action's assignee can delegate action to someone in his team as a secondary action owner. Once the secondary owner completes the action it again returns back to the primary owner for final closure where the final closure of the action can only be performed by the primary owner only.
How to delegate action items to other users?
From the action page, select the action item, and click on more options on the quick view. click on delegate action
Start typing the secondary action assignee name or email and click on delegate.
On the action's quick view you can view the discussion and activity log.