RadExpander is a very simple HeaderedContent control - having just a header and a content and a single action for toggling the content visibility. When you insert several expanders into a single container the expander do not communicate with each other.
On the other hand the PanelBar is ItemsControls which supports hierarchy, databinding and has much more complicated functionality.
That said - you are right that if you place several expander elements into a stack panel and add some additional logic you can achieve similar to PanelBar UI.
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