One possible solution is using Blit
method in WriteableBitmapEx. This method copies (blits) the pixels from the WriteableBitmap source to the destination WriteableBitmap (this) and following is a simple sample.
var writeableBmp = new WriteableBitmap(1, 1);
var image1 = await writeableBmp.FromContent(new Uri("ms-appx:///Assets/image1.png"));
var image2 = await writeableBmp.FromContent(new Uri("ms-appx:///Assets/image2.png"));
image1.Blit(new Rect(0, 0, image1.PixelWidth, image1.PixelHeight), image2, new Rect(0, 0, image2.PixelWidth, image2.PixelHeight));
//BlendedImage is a Image control in XAML
BlendedImage.Source = image1;