Bringing up a good old question. I agree with all that you can assign more than one resource per task. You certainly don’t want to create multiple tasks in order to emulate the need. I suspect what it was trying to communicate (as a best practice) is that the more people you assign to multiple tasks, and harder it is to manage overall resource utilization. It is much easier to arrange tasks with a single assigned resource in order to manage overallocation than it is to manage many tasks with multiple assignments. That is one of the concepts to consider when initially creating and assigning the WBS. But, it is only one of many considerations and not ever an absolute.