I haven't been to either water park before, but we are going this summer in July and I plan to do both water parks the same day. The way I am planning we will have breakfast at our resort then head to BB, we'll stay there for about half the day (until around lunch time or after lunch), then we'll head over to TL and spend the rest of the afternoon there (until 5 maybe...depending on how much we can take). After that it's back to the hotel to change and go to dinner. Then spend the night at the hotel relaxing by the pool/hot tub or in the room.
I know it sounds so busy, but we've spent all day and night in MK before in the summer heat, I'm hoping the water parks can't be worse than that.