Typhoon Lagoon + DHS in a day OR Discovery Cove and SeaWorld in a Day
I vote for TL and DHS in one day if your main goal is two parks in one day. I would open TL and be done in about 4 hours, change up and head to DHS staying to enjoy the night time shows. One coaster, one drop thrill ride.
Discovery Cove is a full day experience, particularly for the cost. We open and close it, then have a nice dinner somewhere. I would never give up a minute for another park. SeaWorld is spread out and lots of walking so you need to commit some time if you are going to get much done, if you want to see all the shows etc.
SeaWorld gives them 3 coasters and the rest have the animals and low key stuff. If you mean Christmas Holidays SeaWorld's are the best. If you lean that way, get to Discovery Cove at opening trying to finish up around 3, head over to SW for the holiday stuff. Let your coaster person go off on their own. Plan to stay to the end, as the night time is beautiful. It should be included in your DC ticket so if you are only there a few hours and don't do much, it didn't cost you anything.