Material resources as set out by Project do not have assignment units. You can change the type of the material resource to Material - but it sounds like your "material" resource is a work resource.

Work resources perform work - not material resources. It sounds like you have things backwards. If Resource A is performing the task, they cannot be assigned at 0%.

Please tell me why you are doing what you are doing. I understand you wish to see that the work resource is assigned to both tasks in team planner - but with zero assignment units, that is not going to happen. The Team Planner view shows you work resources who are assigned to tasks and have a work value.