How to set TabIndex to a ComboBox within a ContentControl?

I have following code -

<ContentControlKeyboardNavigation.TabIndex="6">//Point A
                           <ContentControl.Template>
                               <ControlTemplate>
                                   <ContentControl x:Name="content">
                                       <ContentControl.Template>
                                           <ControlTemplate>
                                               <ComboBoxStyle="{StaticResource ComboBoxStyle}"ItemsSource="{Binding Path=Property1}"
                                                 Margin="0, 7">
                                                   <ComboBox.SelectedValue>
                                                       <BindingPath="PropertyText"/>
                                                   </ComboBox.SelectedValue>
                                               </ComboBox>
                                           </ControlTemplate>
                                       </ContentControl.Template>
                                   </ContentControl>

                                   <ControlTemplate.Triggers>
                                       <DataTriggerBinding="{Binding Path=Property1}"Value="val1">
                                           <SetterTargetName="content"Property="Template">
                                               <Setter.Value>
                                                   <ControlTemplate>
                                                       <ComboBoxStyle="{StaticResource ComboBoxStyle}"ItemsSource="{Binding Path=Propert2}"
                                                         Margin="0, 7">
                                                           <ComboBox.SelectedValue>
                                                               <BindingPath="val2"/>
                                                           </ComboBox.SelectedValue>
                                                       </ComboBox>
                                                   </ControlTemplate>
                                               </Setter.Value>
                                           </Setter>
                                       </DataTrigger>
                                   </ControlTemplate.Triggers>
                               </ControlTemplate>
                           </ContentControl.Template>
                       </ContentControl>

Explanation

See the Point A at first line of code. The TabIndex is working fine, but if I move this to internal elements like ComboBox, it goes to next TabIndex. Also I have a trigger which changes Template on corresponding trigger.

Requirement

Whichever Combobox is visible should get TabIndex="6".

The parent control i.e. ContentControl should not get any TabIndex.

Please suggest some solution.

I have tried KeyboardNavigation.TabNavigation property but is not providing any solution.