I have modified your project as follows:
1. The part that combines the 2 images with a white background is correct, so I did not change it.
2. The result image needs a suitable alpha bitmap. So I added the following code to combine the 2 alpha channels from the source bitmaps into one alpha channel. This new alpha channel gets added to the target image before saving it:
//Before the loop:
RasterImage alphaCombineImage = RasterImage.Create(1024, 1024, 24, 1024 * 1024, RasterColor.Black);
Leadtools.ImageProcessing.GrayscaleCommand gs = new Leadtools.ImageProcessing.GrayscaleCommand(8);
gs.Run(alphaCombineImage);
RasterImage rasterImgAlpha = null;
//Inside the loop:
rasterImgAlpha = rasterImg.CreateAlphaImage();
command.MaskImage = rasterImgAlpha;
Leadtools.ImageProcessing.CombineFastCommand combFast = new Leadtools.ImageProcessing.CombineFastCommand(
alphaCombineImage, new LeadRect(0, 0, alphaCombineImage.Width, alphaCombineImage.Height), LeadPoint.Empty,
Leadtools.ImageProcessing.CombineFastCommandFlags.OperationMaximum);
combFast.Run(rasterImgAlpha);
//After the loop, before saving:
combineImage.SetAlphaImage(alphaCombineImage);
I'm attaching the modified project