It's not so much that people are referring to the two water parks in the area, the more the merrier I'd say, but it's that they're both operated by the same company, If I the consumer were to buy a combo pass, I intend to visit both SFoG and WW. With the addition of Hurricane Harbor, why should I even consider the option of a pass to both parks when I can just visit SFoG/HH for the price of a regular season pass.
I personally live between the two parks, but why should I now make the drive to White Water when I can just exclude that cost from a regular season pass. Nothing there convinces me to spend more money.
If these wer operated by two separate companies then the two could seriously compete with each other, drawing the same waterpark-seeking crowd to whichever was the better park. Why would SFoG now compete with themselves?