WPF TabControl in another TabControl [doesn't work, Why?]

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WPF

C#5.0

TabControl

Hello there,
I have a TabControl, where one of it's items should contain another TabControl.
I already read that this was only possible with a workaround in WinForms, but I am using WPF. Nevertheless I have problems to implement that. Here's a scheme of my XAML-Code:

<Window>
  <Grid>
    <TabControl>
      <TabItem/>
      <TabItem>
        <TabControl>
          <TabItem>
            CONTROLS SHOULD BE HERE
          </TabItem>
        </TabControl>
      </TabItem>
    </TabControl>
  </Grid>
</Window>

The Controls of the TabItem aren't displayed, only if I place them into the first Grid, they are viewable. It still doesn't work when the TabItem has a Grid/Stackpanel.