Here is C# code that shows how to add pages of one TIFF file to another: 
namespace UserGuide.Programming.Tiff
{
    class MergeTIFF
    {

        public void MergeTiffFiles(System.IO.Stream firstStream, System.IO.Stream secondStream)
        {
            // open the first TIFF file
            using (Vintasoft.Imaging.Codecs.ImageFiles.Tiff.TiffFile firstTiff = 
                new Vintasoft.Imaging.Codecs.ImageFiles.Tiff.TiffFile(firstStream))
            {
                // open the second TIFF file
                using (Vintasoft.Imaging.Codecs.ImageFiles.Tiff.TiffFile secondTiff = 
                    new Vintasoft.Imaging.Codecs.ImageFiles.Tiff.TiffFile(secondStream))
                {
                    // for each page in the second TIFF file
                    for (int i = 0; i < secondTiff.Pages.Count; i++)
                    {
                        // add page of the second TIFF file to the first TIFF file
                        firstTiff.Pages.Add(secondTiff.Pages[i]);
                        // save changes to the first TIFF file
                        firstTiff.SaveChanges();
                    }
                }
            }
        }

    }
}