Which in turn would be good for the lungs because they'd adapt to it. You can modify your lungs you know. Swim long enough and do a lot of diving and you'll be able to hold your breath a lot longer than somebody who isn't a diver. You'd also be able to swim and move for longer underwater over somebody who isn't a diver. And, there's no reason that the secondary heart isn't a full one. Let's just say that it's a slower, less powerful secondary heart. Enough to give a boost but not to increase the strain exponenitally.