Hi @SidneyB ,
Are you using a SharePoint task list?
Do you want to mark the single task completed when one of the assigners close the task?
If yes, this is the default feature in both Planner and SharePoint task list, that when one of the assigners complete the task, the task is completed.
Below flow may be helpful to you.
Here is the flow to create a planner task with information from SharePoint task:


Besides, if you want to edit the task in SharePoint when the same name planner task is completed, you can create another new flow. More detailed information, you can refer to this template flow:
https://flow.microsoft.com/en-us/galleries/public/templates/32ed5782ce15419ab0a551fa0a51e6be/update-an-item-in-a-sharepoint-list-when-a-task-is-completed-in-planner/
Note that in the condition, we need to use the TaskName column from SharePoint to compared with Title value from Planner task, if the condition is true, mark this SharePoint task as completed.

Best regards,
Allen