recurringComboBox.xaml

            <ContentControl Content="{Binding}">
                <ContentControl.Resources>
                    <Style TargetType="ContentControl">
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding SelectedItems.Count}" Value="1">
                                <Setter Property="ContentTemplate">
                                    <Setter.Value>
                                        <DataTemplate>
                                            <ComboBox />
                                        </DataTemplate>
                                    </Setter.Value>
                                </Setter>
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </ContentControl.Resources>
            </ContentControl>