Hi,
Since 6 months ago, HikaShop now supports external images.
So when you add an image to a product, instead of uploading the image or entering the path of the image in the upload folder, you can also specify a URL.
So use the upload system on one website, and then enter the URL of the image on the other website and you'll be set.
Otherwise, without changing anything to your website or to HikaShop with the "half the way" solution you already have, all you need to do is to create a simbolic link between the upload folder on both websites since they are on the same server and that should do.
You'll have to properly set the permissions and it will work. Unfortunately, we can't help with that. It's not a matter of changing code in HikaShop but for your server administrator to properly configuring the symbolic link on the server.