I am not sure how they do it now, but when we bought the parking from DC, I picked up a card at DC that I used to show the parking attendants at the other parks. It was good for 14 days from the date we picked it up. I had to ask for it where I checked in at the DC. We used it to park twice at the three parks, BGT, Aquadica and SW. Well worth it for us. Oh, and if you go to Aquadica, and then want to go to SW, just save your parking ticket they give you when you enter Aquadica. It's good for the same day parking at SW or vice versa.
Have a great time. We really enjoyed DC and the other Busch parks.