I agree with you Ralph  but requirement is different.
Currently, we can create multiple template for Test Plan but can assign only one at a time. We have two different teams want to create Test Plan with their own template without disturbing each other.
I don't think there is any way to give choice to user which template he wants to use at the time of creation of Test Plan. so we though, would be better if we create separate Test Plan/work item. So both teams will use their own Test Plan.