I would go the route of making seperate templates for things that are supposed to look the same like this:
<DataTemplate x:Key="sharedTemplate">
<StackPanel>...</StackPanel>
</DataTemplate>
<HierarchicalDataTemplate DataType="{x:Type InnerType}"
ItemsSource="{Binding Items}">
<ContentControl Content="{Binding}"
ContentTemplate="{StaticResource sharedTemplate}" />
</HierarchicalDataTemplate>
<HierarchicalDataTemplate DataType="{x:Type OuterType}"
ItemsSource="{Binding Items}">
<ContentControl Content="{Binding}"
ContentTemplate="{StaticResource sharedTemplate}" />
</HierarchicalDataTemplate>
There are architecturally more elegant solutions out there, but seeing look and feel is handled by designers I don't like to use solutions that are too complex programming paradigm wise in these things.