public class LoginDataToBoolConverter : IMultiValueConverter
  {
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
     return ((string)values[0]).Length > 0 && ((string)values[1]).Length > 0;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
      throw new NotImplementedException();
    }
 }

<Button.IsEnabled>
    <MultiBinding Converter="{StaticResource MyConverter}">
        <Binding ElementName="UsernameTextBox" Path="Text" />
        <Binding ElementName="PasswordTextBox" Path="Password" />
    </MultiBinding>
</Button.IsEnabled>