two itemtemplates for one listbox

You can define DataTemplates that apply to any instance of a specific type by specifying the DataType without an x:Key. Using this method you don’t assign anything to ItemTemplate – the templates are applied automatically.

<ListBox ItemsSource="{Binding Path=MixedList}">
    <ListBox.Resources>
        <DataTemplate DataType="{x:Type local:BananaViewModel}">
            <TextBlock Text="{Binding Name}" Foreground="Yellow"/>
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:AppleViewModel}">
            <TextBlock Text="{Binding Name}" Foreground="Red"/>
        </DataTemplate>
    </ListBox.Resources>
</ListBox>