Personally, I can't think of any legit reason why someone would need to know every site that someone else owns. That just sounds a little ominous.
If it is a legit need - ask the individual outright. If you can't ask the individual outright, then you certainly have no business noseying about their personal information. If you are going after someone who is being sneaky, chances are their whois is different on multiple sites anyway, of no help to you.
So, I guess the question I would throw back at you is this: Why do you want that information?