WPF:同时在两个组合框上运行验证程序

我有以下两个包含类似项目列表的组合框。 两个组合框不能具有相同的值。

            <ComboBox Name="OldEmpNoListBox"
                      IsReadOnly="True"
                      ItemsSource="{Binding OldEmpNoCollection}"
                      SelectedValue="{Binding Model.FieldNameForOldEmpNo, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" 
                      HorizontalAlignment="Left"
                      Width="150"
                      IsEnabled="{Binding HasItems, RelativeSource={RelativeSource Self}}"
                      Grid.Row="1" Grid.Column="1">
                <ComboBox.Text>
                    <Binding Path="Model" 
                             UpdateSourceTrigger="PropertyChanged">
                        <Binding.ValidationRules>
                            <vm:FieldNamesValidator ValidatesOnTargetUpdated="True" />
                        </Binding.ValidationRules>
                    </Binding>
                </ComboBox.Text>
            </ComboBox>
            <ComboBox Name="OldCompanyListBox"
                      IsReadOnly="True"
                      ItemsSource="{Binding OldCompanyCollection}"
                      SelectedValue="{Binding Model.FieldNameForOldCompany, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" 
                      HorizontalAlignment="Left"
                      Width="150"
                      Grid.Row="2" Grid.Column="1">
                <ComboBox.Text>
                    <Binding Path="Model" 
                             UpdateSourceTrigger="PropertyChanged">
                        <Binding.ValidationRules>
                            <vm:FieldNamesValidator ValidatesOnTargetUpdated="True" />
                        </Binding.ValidationRules>
                    </Binding>
                </ComboBox.Text>
            </ComboBox>

这是验证器：

/// <summary>
/// Duplicate Values Validator
/// </summary>
public class FieldNamesValidator : ValidationRule
{
    /// <summary>
    /// Validate OldEmpNo field name does not match Old Company field name
    /// </summary>
    /// <param name="value"></param>
    /// <param name="cultureInfo"></param>
    /// <returns></returns>
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        EmpNoOptionsSettingsModel _empNoOptions = (App.Current.Resources["Locator"] as ViewModelLocator).ParametersEmpNoOptionsViewModel.Model;
        // if OldEmpNo field name = Old Company field name
        if (_empNoOptions.FieldNameForOldEmpNo == _empNoOptions.FieldNameForOldCompany)
        {
            return new ValidationResult(false, "Field name for Old Emp No cannot be the same as the field name for Old Company");
        }           
        return ValidationResult.ValidResult;
    }
}

验证按预期工作，只是它仅在当前正在更改的任何组合框上运行。 例如，如果组合框具有相同的值，则每个框都以红色勾勒出轮廓。 我更换了第一个盒子，红色被移除了，但红色仍然留在第二个盒子上。 有没有办法让两个组合框在其中一个被更改时验证自己？