Tabs are not normally selected while dragging docking windows around (before releasing the drag), as that would be confusing for the end user, especially when drag drop targets and dock guides are showing for the movement.
That being said, I understand how in this nested/linked dock site setup you have, you would want the ability to do that. Unfortunately nothing is built in to support that at this time.
In the next build, we will add a new protected virtual DockSite.NotifyWindowDragMove(Point location) method that will receive a screen coordinate as the pointer moves during a docking window drag operation. You will be able to override that on DockSite and add your own logic, but be sure to call the base method implementation first since it controls the dock guides, etc. This will at least give you an extension point for where you can plug in logic to implement what you're trying to do.
In your NotifyWindowDragMove logic, you could take that Point location and do some WPF Visual hit testing on your end to see if you are over a tab that requires selection and then programmatically select it.