I've done that for years and it seems OK, though from what I've read recently about battery management systems it may not be optimal - optimal would be combining individual 2V cells with intelligent DC-DC converters and smart chargers, never letting any one cell fully discharge or overcharge, rather than just connecting a set of 6 cells in series to an engine alternator as has been normal practice for decades.
But on optimal solution is probably still too expensive; it's cheaper to just replace batteries every few years. It's OK to connect two 12V wet lead-acid batteries in parallel, though they might be from different manufacturers and different Ah capacities. I'm fairly sure it's bad to connect different chemistries in parallel, e.g. lead-acid and lithium-iron, and I believe that mixing AGM and wet lead-acid is not that good either as they have different charging characteristics. I have a wet "house battery" and a wet "starter battery" in parallel with a switch, which are connected together to maintain charge with the engine running and split to maintain a starting reserve when sailing or anchored. If one gets highly discharged, it should be charged in isolation until it roughly matches the voltage of the other to prevent a large initial current between them.

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