You want two hearts? The Doctor has two hearts! Let's get two hearts!
Smaller lungs, meaning diminished air capacity unless the lungs have greater or more efficient oxygen transference.
Smaller stomach. This isn't as silly as it might sound. Humans today employ bariatric surgery to make the stomach smaller as a weight-loss procedure. In fact, humans (at least, Americans... right?) can easily live on less food than they do. At worst, your Goliaths would need to eat a bit more often (Elevenses, anyone?)
Smaller intestines. This might be the most believable of the lot. While a small increase in the efficiency of the intestines, particularly the large intestine, would allow you to chop a chunk of them off, I believe it's entirely realistic based on the fact that what makes the intestine efficient is the surface area inside the intestine. A larger person might imply a larger intestine — which you can interpret as either normal-diameter but longer or greater diameter but the same length compared to a normal human. In other words, I think it's entirely believable for the space you need to exist without expecting the intestines to work differently at all.
Your giants never had an "appendix" in the first place. Unlike humans, that space was given over to a second heart. I've changed my mind — I think this is the best choice.
Splitting the load isn't as simple as adding a second pump. If you think about it, two pumps in series means (simplistically) twice the blood pressure in the rest of the system. That's most efficient from a need-for-internal-space perspective, but it has other problems you might consider. Capillaries must be designed for that pressure (or your giant's eyes are perpetually blood shot or simply blood red). You might need a thicker dermis to ensure capillaries and veins don't burst through the skin or serious (and I mean serious) varicose veins. Yes, you can design (or declare) your Goliath to handle blood pressure. Or...
You can put the pumps in parallel, meaning one of two things: (a) you either have a more complex cardio-vascular system such that, say, blood to one half of the body goes through one heart and blood for the other half of the body through the other heart, or (b) blood pressure increases.
Series vs. parallel solves different kinds of problems. Series is useful when you need to have the same pressure over a long distance. Parallel is useful when you need more pressure. A series solution will not drain the proverbial pool faster, but it will let you push the drainage a greater distance. Parallel will drain the pool twice as fast, but you can't significantly push the water any further than with one pump (putting the output of the two pumps in a single, small pipe increases the pressure, but decreases the volume).
You see, blood pressure is a tricky thing. It's not completely understood by doctors today, but they understand a lot of it. Blood pressure is affected by at least:
- blood vessel diameter
- blood vessel wall permittivity
- the volume of blood in the system
- the volume of water in the system
- the force of fluids pumped through the system
- receiving cell permittivity
- chemical control/feedback of the process
Which means "why" might be important
So, you want two hearts! No problem! But you might want to step back for a moment and see if there's room in your story for why you need two hearts. The answer to that question has a fair impact on how the hearts are configured, which will impact how much space you need for the two hearts.