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.