Peukert scoffs at overloaded or under-specced 12v systems--to my mind this is the whole point of migrating to 24v. Therefore I would NOT set aside a dedicated 12v battery. Rather I would either install an OVER-spec 24/12 dc/dc box (my pref as long as it was MUCH bigger than I thought I needed), or I would make sure my 12v tap to the 24v batt bank was running thick wires.
Remember, you're now pulling 12v from half the previous Ah reserve, so Peukert is starting to chuckle.
On my current system, I run 4/0 (four-ought) "mains" from the bank to the inverter. Because this is such a beefy path, I tap most of my 12v from busbars at the inverter lugs. If I went to 24v, rather than lay a new set of thick cables to the bats, I would tap my dc/dc converter in at the end of the 4/0 run.