It sounds like the most common way to do these things is to have one company operate one gambling and one non-gambling site and just tell people they operate the other site on each. No money's changing hands, so that's not advertising. Then you can advertise to go to your non-gambling site, and they can organically navigate to the gambling site which was disclosed, not advertised. You would almost have to ban companies which have any interest in a gambling product from advertising anything at all.