Kind of obvious for both of them though: evolving two hearts is putting two pumping systems in parallel on the same fluid reservoir. Given how the pump works (evolution can't handle making gears in general), you'd need a precise synchronization system to keep everything working properly - you're basically much more likely to die from the "high availability" system then a very robust single system (and the usual proviso: evolution doesn't need you alive, it needs you to reproduce and the offspring have some decent chance of survival. Percentage survival rates are just fine.)

Same for the brain but I'd say writ large: easier to make an extra human then try and coordinate a redundant brain.

Those ones are much easier to answer: they require very good coordination between each other. "Split-brain" is literally a canonical failure mode in the study of distributed systems, for example. (We kind of do have two brains, right, linked by the corpus callosum; there's a reason they're so close together.)