You wouldn't have to be a hacker to know if two domains are related. If a hacker that got into your filesystem they could figure it out in a few seconds or so, or if you're using something like an analytics software that ties both domains together, using the same affiliate ID on multiple sites, re-using images or CSS files from one site on another, etc. They will have the same IP address, but if there are others on the same IP address and the domains don't look or sound alike, most people wouldn't know or care. Not sure what your goal is in keeping the sites separate, though.