That's too bad that sea world closes at 6pm when you're there. If it closed at 9pm or 10pm, I'd say absolutely you could do both. We did both in March, but SW was open until 10pm. We left DC at 6pm and did 3 hours at sea world - and in that amount of time saw everything that we'd wanted to see.
I agree with pp, since discovery cove isn't a touring park you will probably WANT to be there all day. Before going, I too thought maybe we'd feel finished at 4pm and head over to SW earlier, but nope, once we were there we stayed in the water right until 5:30pm when they closed things down (and even then we weren't ready to leave yet)! Discovery Cove is amazing!
But if you really want to see sea world too and are disciplined in making yourself leave discovery cove, of course you can do both in a day, if that's the only way it'll work.