As previously mentioned achieving drag and drop between two separate RadDocking instance is not possible in the current version of the control.
Unfortunately implementing such functionality is not a simple task and there are many issues that could be hit along the way. Because the control is not a simple control and many of its parts are connected to each other by either being a parent or child, implementing such functionality where one Pane/Group is docked outside of its parent Dock borders is not a simple task.
Implementing such functionality is on our TODO list and if there is enough demand for it we will do our best to have it as a built-in functionality in the control as soon as possible. You can keep track of the feature request's status here
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