using Syncfusion.DocIO;
using Syncfusion.DocIO.DLS;
//Open source document stream
using (FileStream sourceStream = new FileStream("SourceDocument.docx", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
    //Load source Word document
    using (WordDocument sourceDocument = new WordDocument(sourceStream, FormatType.Automatic))
    {
        //Open destination document stream
        using (FileStream destinationStream = new FileStream("DestinationDocument.docx", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
        {
            //Load destination Word document
            using (WordDocument destinationDocument = new WordDocument(destinationStream, FormatType.Docx))
            {
                //Import the contents of the source document at the end of the destination document
                destinationDocument.ImportContent(sourceDocument, ImportOptions.UseDestinationStyles);
                //Create output file stream
                using (FileStream outputFileStream = new FileStream("Result.docx", FileMode.Create, FileAccess.ReadWrite))
                {
                    // Save the Word document to the output file stream
                    destinationDocument.Save(outputFileStream, FormatType.Docx);
                }
            }
        }
    }
}