CreateIcon(1,{{Type=#File, Image="image.png"}}) => good work
CreateIcon(1,{{Type=#File, Image="image.png", SelImage="Selimage.png"}}) => No good work => 2 pictures side by side in icon
Hmm, how did you create this icon? Using SaveIcon()? That currently doesn't support Amiga icons but saves icons in a special, proprietary format based on PNG. From the doc:
Note that although Hollywood icons are normal PNG images, they contain additional metadata which is why you mustn't edit them with your favourite image manipulation tool because that might lead to the loss of said metadata. Hollywood icons should only ever be created by using the SaveIcon() function.
If you want to make a "dual PNG icon", which works on MorphOS and at least with some 3rd party extensions on OS3 and OS4, you can just join two PNG images together by this shell command: Join image.png Selimage.png AS MyIcon.info
You can naturally execute that kind of command lines from Hollywood too.