The problem comes in, what happens when a heart fails? depending on the failure mode, it may represent a total blockage, in which case you’re toast. You might be able to survive with one heart if you had two, but if you add a second heart, then your other heart will likely be less developed unable to perform at whatever peak performance you had before.

If your method of redundancy adds more single points of failures. Also, the addition of a second heart poses the problem of keeping them coordinated; with all sorts of problems coming up if they get out of sync. adding redundancy will always add complexity, especially as you work to remove single points of failure and try not to add extra. In some systems, it’s just unwise to add redundency because the complexity means it’s more likely to fail.

Famously, Charles Lindbergh, for example, opted for a single reliable engine over two engines. It kinda flew in the face at the time. But then he was the first to go from NY to Paris in a non stop flight, in the Spirit of St Louis. Similarly, we can expect, if there was in fact some significant advantage, that then, everybody would be doing it. Or, at least, lots.

Keep in mind, cephalopods have 3 hearts- 2 are single chamgered things that boost blood over gills, and the 3rd provides bloodflow to the rest of the body. Hagfish have one chambered heart and several boster things that aren’t really much of a heart. Earthworms aren’t possessed of true hearts (they lack chambers and valves,) cochroaches and leaches also don’t have true hearts.

But where we see 4 chambered hearts (birds, mammals, and crocodillian reptiles,) they all only have 1. That should tell you something.