Hi Mark,
this is not possible. The path transition, along with all actions, is only triggered when the task is completed. If you set it up, so that all needs to complete the task, you will have the described effect.
An alternative approach could be to use the standard "if anyone completes the task" definition and add a flow control step. In this step you can check whether all required persons have completed the task and move on. If this is not the case, you can execute a path which moves the workflow back and reassigns the tasks for those who have not completed it.
This post can give you a starting point for getting the task for your step and the people who have not completed the task.
https://community.webcon.com/forum/thread/1691?messageid=1691
If you can wait for the item list update and you will update the item list with the date when the person completed their task, you could retrieve it from the WFElementTasks table.
Best regards,
Daniel