Can we use our season pass from Busch gardens if we add the fun pass to it for both parks and just pay the difference

No you need the pass for both parks

Yea like Jamie said you have to have the 2 parks seasin passes. Bush Gardens and Adventure Island

Not if its only a Busch Gardens pass. You might be able to use it for parking.

No

You have to have the Bush gardens adventure island combo pass :)

Only if you have the fun card or pass good for both. It would say on your card.