That is correct, you can not use two days of admission on the same day.
It's a hard and fast rule. The only way to visit two Disney parks on the same day using Magic Your Way tickets is with the Park Hopping option.
You can always add the park hopping option before the ticket expires 14 days after first use. On a 4 day ticket this is an additional $45+ tax per ticket.
Now, a suggestion would be if you don't want to spend the extra for park hopping, pick the day that has the longest operating hours at Disney and go to Sea World first thing in the morning. Come back in the evening and then use the "extra" day at Disney that night.