Presumably you want to have 2 banks separate for discharge purposes so that one bank is always good for engine starting. This leaves you with in each bank 2 batteries in paralell in series with another 2 batteries in paralell. Or 2 battweries in series in paralell with 2 batteries in series. I don't think it matters much which ie if they are only paralelled at the 24 volt point or the 12 volt point as well. Possibly the latter would give better performance if one cell fails but I can't be sure. Then again maybe you should have 4 separate banks to discharge individually (presumably charge in paralell) this might give the most flexibility but would take more carefull monitoring. The bottom line I dunno will
The benefit of a 24V system is reduced current so the wiring can be in thinner cable. The downside is that many of the 12V goodies you can buy are not available in 24V and, of course, the caravan and RV market is all 12V. So if it was me, I would parallel the batteries. Most people need or like to have a dedicated starter battery. These days a dedicated purpose-designed starter battery is becoming common, leaving all the big batteries in parallel for the domestic bank. Starting from scratch, that's what I would do.
However, if you have an auxilliary generator you might consider sharing an engine cranking battery between the two engines and carry a pair of ordinary jump leads in case of emergency.
Good point about the battery failure issue.
If you get a dud cell and you had set up as 4 pairs of 24v 110a banks, you can take one of the banks out to prevent further damage to the other cells and only loose 25% of you capacity.
If you have parallelled as two 440a banks and have lost one cell, you could disconnect it, but you'll be putting the whole bank out of equalisation causing more stress to half the bank and more work to equalise when you replace your cell.
I had found no real diference either way till you mentioned this point
When a cell fails catastrophically it goes either (nearly) short-circuit or open-circuit. By 'cell' we are talking about one of the six cells that make up a 12V battery. When a cell fails open-circuit in a battery the whole battery appears dead - just as if it has been removed. This may go undetected for a long time in a bank of parallelled batteries but will be immediately obvious if the battery is in series with another. The worst case consequence is power failure.
If one cell fails short-circuit then the terminal volts of the battery fall by around 2V and the charger (engine or mains battery charger) is liable to seriously over-charge the remaining cells. This results in gassing, temperature rise and even boiling cells; very serious stuff which can lead to total power failure, leaking acid into the bilge, fumes or even fire. This also happens with sealed batteries and gels and can result in deformation of the battery case. The only way to detect this from the instruments is by noticing that the batteries are taking charging current when you wouldn't expect them to be. Then, if all the batteries are in parallel, feel all the batteries and the battery with the shorted cell will feel warm. Isolate that one battery and you are back in business - albeit with reduced total Ah capacity.
I recently had a shorted cell in a 440Ah gel battery bank made up from 2 220Ah gel batteries in parallel. I noticed on my battery monitor (I have a sophisticated battery monitor similar to the Link 10) that the batteries were taking 3A on charge long after I would have expected the charge to have returned to zero. I felt the batteries and one was very warm to the touch, so I isolated that battery. I shudder to think what might have happened had I not noticed - which I wouldn't have had I not installed the monitor a couple of months before, or had I left the boat for a few weeks. /forums/images/graemlins/shocked.gif