You can add a second day for $10, no matter which ticket you buy, even if it is a discounted ticket.
We added a second day in Dec to a $50 week day ticket. Went Friday for the week day part. Then added the second day for $10 once there and went back on Sunday. Can't beat two days for $60.
If you aren't sure if you need 1 or 2 days, just get the a one day ticket, or get the Sea World / Busch Gardens combo, then if you don't finish in one day, go to guest relations and have the second day added. Honestly, unless you want to visit both parks, I'd hold out on buying any ticket until the last minute in the hope that some type of discount is offered before you leave. There aren't any great 1 day discounts right now, except the Busch Gardens deal. So if Busch Gardens doesn't interest you, I'd just wait. You shouldn't be an any danger of regular tickets selling out.