There's no way that a human heart would be able to pump enough blood to supply the full vascular needs of a thousand pound centaur.

Most likely, the human halfs heart is more of a vestigial organ or more of an assisting organ to help ensure that even under strenuous cardio that the brain receives enough blood flow to operate optimally.

The lung system though is definitely a weird one. I would be more inclined to believe that there are no lungs in the centaurs human half, maybe the areas that are lungs in a human would be something like a cud sack or some sort of extra biliary devices to increase the amount of acidity to break down their largely herbivorous appetite.

Or twice as vulnerable.

Centaurs are Klingons. Got it.

Centaur Timelords confirmed.

Bold assumptions in there.

Nothing says there have to be duplicated organs at all. You wouldn't necessarily even need two sets of lungs for them to be able to speak, just an air bladder that gets filled up without doing any major oxygenation. You could even have the "horse" lungs doing all the work for that too, without the need for a secondary air sac.

You could even do away with a full skeletal system in the upper section. Could be almost all muscle around some bones for support and mobility.

All that would be necessary for the upper part are digestive tubing, breathing tubing, and a skull on top of a section of something akin to vertebrae for turning the head itself.

Think of that human torso being a very complicated neck rather than a half of a human grown onto a horse.

Now, I would think that with the extra space, there would be something like a stomach in there to begin breaking things down before sending them along but that isn't mandatory for the basics to work.

Now, where things get interesting is reproduction. There's where you'd see some trippy arrangement to allow for birthing a baby. I've always thought that maybe the "human" part isn't developed the same. More of a lump of a head with stumpy little proto-arms that finish growing after birth. Just enough so they could run and eat, with the rest of the torso section slowly arising from its flesh prison fully during puberty.

They'd spend their first few years slowly growing the trunk and arms, allowing the muscles to develop over time.

Even if there are ribs, they wouldn't have to be attached to anything. They could just be under the muscles a little to give structure without the need to have organs to protect.

Also, how many sets of genitals do they have? Do the front and back have to match if there's genitals on both? There's sci-fi centaurs where the whole thing is way more complicated than you'd think.

I've seen certain images that depict them having two sets of sex organs as well.

Idk about that, 2 sets of internal organs just means there's twice as many that can fail

If the horse heart fails, the human heart might just be strong enough to keep the rest of the body alive as long as you don’t move much. Probably not a long term solution, but it’s better than nothing.