ViewState is a protected property of each Control.(Its defined in System.Web.UI.Control.)
When you interact with ViewState, you are always inside a particular control's scope and using its ViewState property. So when you say Naming container, you are referring to a control - the UserControl, the Page, etc.
Simply by knowing that ViewState is a protected property, you know that you cannot access it from outside of the control that owns it. In your case, that means one UserControl has its own ViewState that is not visible to any other.
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