1 x 24v always.
2 x 12v in series is just bad planning.
That was typical for lead acid batteries. But these are LiFePo4 and have BMS’s. Unless you want to disconnect them for maintenance every few months. (Which defeats the point of buying batteries that are designed for no maintenance)

The Fenix HL10 is a decent single AAA headlamp (and van be used as an angle light as well).