If Page A is linking to Site X and to Site Y , similarly Page B, Page C , Page D … (some good number of pages) links to both the sites , then you do a G or Y search with , you will get Site X as related (even though X and Y may be extreme apart with theme) and similarly if you do G or Y search for related: you will get related Site Y, thus Search engine can understand that Y and X are related.This is not a proven theory , but a happening thing as manual checking is gearing up, once big mamma said precaution is always better than cure, be careful to enjoy the investment on your site.