The best way to create multipage tif files is to store each individual image in an ImageCollection instance. Then call the Save() method which has several overlaods. The easiest one is the Save method in which you pass the path and filename to be saved. For example:
// Create new instance of an ImageCollection
ImageCollection myCollection = new ImageCollection();
// Add images to the collection (this is just one of many ways to do so)
myCollection.Add(ImImage.Load(@"C:\temp\images\firstImage.png"));
myCollection.Add(ImImage.Load(@"C:\temp\images\secondImage.png"));
myCollection.Add(ImImage.Load(@"C:\temp\images\thirdImage.png"));
// Save the collection to a single tif image
myCollection.Save(@"C:\temp\images\allImages.tif");
Of course, if you are using the Viewer control, the Images property is an ImageCollection conveniently, so you can simply call:
viewer1.Images.Save(...);