I am afraid that this functionality is not supported with the radTreeView. However, you can try to custom implemented it, using jQuery. For example you can try to cancel the propagation of the click event of the Expand toggle and hook a dblclick event on the same, so you can use the ClientAPI of the control and manually call the expand on the specific node.
However, please keep in mind that this is not a supported scenario and subsequent issues may arise.
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