(BTW, you freaked me out a little with "Busch Gardens San Antonio". I think you meant "Sea World".) :D
BUT, there is no other rules -- you don't have to be related, in the same house, or even in the same state. So if you have 3 other people you know who want passes, and you can work out the money, you can save a good bit of money by going together on the passes.
What I find more confusing and kind of stupid is the wide variety of "savings" they offer for current passholders.
Looking at BGE again, you can save almost 20% off the normal price of a pass if you are an existing passholder -- for the 2-park pass. But for the Platinum pass, you only save 7%. Throw in the 12-month rule instead of the calendar-year rule, and the "continue month-by-month" you get with ez-pay, and there are simply too many decisions.
Normally I would have paid for next year's single-park pass already, knowing that I could upgrade to a 2-park or platinum if necessary. But they actually TOLD me not to buy my passes yet, but to wait until spring because of the 12-month rule. And now I'm not sure WHEN if ever I will buy.
And I had to do a spreadsheet just to compare all the offers. And that was just for BGE -- I didn't even LOOK at the other parks.
I know that if I decide for a Platinum pass, there is no point it hurrying to get my passholder discount. My family of 4 will be $242 each for 2-years; If I wait too long, it will be $254, or $12 more. Since the cost per month is $10, if I wait 2 months I actually save money even without the passholder discount.
I can't imagine why a company would make a plan that actually encourages it's current passholders to NOT send them money.
If i could change one thing for them, it would be to offer the passes so that the start date is the first time you go to the park, rather than the day you buy.
The way it is, I will wait until the week before I know I am going, and buy my passes. If they offered a pass that started at the 1st visit, I'd have already sent them my money.
One year I got the six-flags all-parks pass for $40, by buying it with a $10 discount from Wyandot Lake in Columbus, Ohio (I did have to go there first to get the pass).