I got my tickets direct from DC. You can go to SW, Aquatica, and BG (if you purchase that upgrade) for 14 consecutive days, either starting on your DC day or starting before DC. I did 3 park days during the week before my DC day, and 1 after.
Your DC confirmation e-mail is your ticket if you go to other parks first (and your parking pass).
If you plan on driving and doing more than 1 day at SW or Aquatica, it is more cost effective to upgrade 1 ticket to the 3 parks which includes free parking, even if you aren't going to BG (I think it's about $25 to park, and the upgrade is $25).