RadDocument radDocument1 = RtfSourceProvider.Import(rtf1);
RadDocument radDocument2 = RtfSourceProvider.Import(rtf2);

AppendDocumentOptions options = new AppendDocumentOptions();
options.FirstSourceSectionPropertiesResolutionMode = SectionPropertiesResolutionMode.NoSectionBreak;

RadDocumentMerger merger = new RadDocumentMerger(radDocument1);
merger.AppendDocument(radDocument2, options);

RichText.Document = merger.Document;

 StyleManager.ApplicationTheme = new Office2016Theme();

  public class MyRenderer : SpreadStreamExportRenderer
  {
      public override void SetCellValue(DataType dataType, object value)
      {
          DateTime date;
          if (value != null && DateTime.TryParse(value.ToString(), out date))
          {
              base.SetCellValue(DataType.DateTime, date);
              var cell = this.GetCell() as ICellExporter;
              cell.SetFormat(new SpreadCellFormat() { NumberFormat = "yyyy-MM-dd HH:mm:ss" });
              return;
          }
          base.SetCellValue(dataType, value);
      }
  }

 spreadStreamExport.RunExportAsync(FILENAME, new MyRenderer(), options);

<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1" 
             xmlns:telerikDialogs="clr-namespace:Telerik.Windows.Controls.RichTextBoxUI.Dialogs;assembly=Telerik.Windows.Controls.RichTextBox"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/System.Windows.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.ImageEditor.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Navigation.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Input.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RibbonView.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RichTextBox.xaml"/>
            </ResourceDictionary.MergedDictionaries>            
            <Style TargetType="telerikDialogs:LineNumberingDialog" BasedOn="{StaticResource RadWindowStyle}"/>
            <Style TargetType="telerikDialogs:LineNumberingDistanceNumericUpDown" BasedOn="{StaticResource RadNumericUpDownStyle}"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>

---- DEBUG ASSERTION FAILED ----
at Telerik.Windows.Documents.Model.RadDocumentEditor.DeleteTrackChanges(Boolean deletePrevious) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\Model\RadDocumentEditor_Editing.cs:line 693
at Telerik.Windows.Documents.Model.RadDocumentEditor.Delete(Boolean deletePrevious) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\Model\RadDocumentEditor_Editing.cs:line 85
at Telerik.Windows.Controls.RadRichTextBox.Telerik.Windows.Documents.Model.IDocumentEditor.Delete(Boolean deletePrevious) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\UI\RadRichTextBox.cs:line 4890
at Telerik.Windows.Controls.RadRichTextBox.Delete(Boolean deletePrevious) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\UI\RadRichTextBox.cs:line 4866
at Telerik.Windows.Documents.RichTextBoxCommands.DeleteCommand.ExecuteOverride(Object parameter) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\DeleteCommand.cs:line 57
at Telerik.Windows.Documents.RichTextBoxCommands.RichTextBoxCommandBase.Execute(Object parameter, Boolean focusCaret) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\RichTextBoxCommandBase.cs:line 316
at Telerik.Windows.Documents.RichTextBoxCommands.RichTextBoxCommandBase.Execute(Object parameter) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\RichTextBoxCommandBase.cs:line 285
at Telerik.Windows.Documents.RichTextBoxCommands.RichTextBoxRoutedCommand.Execute(Object parameter, RadRichTextBox radRichTextBox) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\RoutedCommands\RichTextBoxRoutedCommand.cs:line 57
at Telerik.Windows.Documents.RichTextBoxCommands.RichTextBoxRoutedCommand.CommandBindingHelper.CommandBinding_Executed(Object sender, ExecutedRoutedEventArgs e) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\RoutedCommands\RichTextBoxRoutedCommand.cs:line 17
at System.Windows.Input.CommandBinding.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.Input.CommandManager.FindCommandBinding(Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
at System.Windows.Input.CommandManager.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.UIElement.OnExecutedThunk(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.Input.ExecutedRoutedEventArgs.InvokeEventHandler(Delegate genericHandler, Object target)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
at System.Windows.Input.RoutedCommand.ExecuteImpl(Object parameter, IInputElement target, Boolean userInitiated)
at System.Windows.Input.RoutedCommand.Execute(Object parameter, IInputElement target)
at System.Windows.Input.RoutedCommand.System.Windows.Input.ICommand.Execute(Object parameter)
at Telerik.Windows.Documents.UI.Extensibility.DefaultInputHandler.HandleInput(RadRichTextBox richTextBox, PreviewEditorKeyEventArgs e) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\RichTextBoxCommands\RoutedCommands\DefaultInputHandler.cs:line 54
at Telerik.Windows.Controls.RadRichTextBox.HandlePresenterKeyDown(PreviewEditorKeyEventArgs e) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\UI\RadRichTextBox.cs:line 4612
at Telerik.Windows.Documents.UI.DocumentPresenterBase.HandleKeyDown(KeyEventArgs e) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\UI\DocumentPresenterBase.cs:line 859
at Telerik.Windows.Documents.UI.DocumentPresenterBase.OnPreviewKeyDown(KeyEventArgs e) in C:\PersonalFiles\Git\wpf\Controls\RichTextBox\Core\UI\DocumentPresenterBase.cs:line 805