I went on this excursion back in August (when you could still pair the Turtles with it). This is totally feasible to do as you suggested. Just try to pick one of the earlier Dolphin times so you have enough time. When we went 5 small buses went over at the same time, but we didn't go back on the same one we came in on. As the PP said, when you were ready, you got in line and the next bus from the Disney-sponsored tour company took you back. For us, the wait wasn't long, but we were rather later in the day.
The turtle place is across the street, and although I'm sure you could spend a whole afternoon there, we had a very nice experience in about an hour. There was a 20-minute tour, during which we saw the main areas with the big turtles in them, then it ended up at the touch tanks where you could pick up 1-1.5 foot turtles. They will take a picture of your group with their camera, but you really don't need to wait in that line if you don't want to because all the other touch tanks are right there and after a couple of minutes you can get close enough to pick one up and take your own pictures if you'd prefer. Then we went over to the nursery and saw 5 little turtles that had hatched 5 days earlier. It was worth it, to us.