Or alternatively...
Yes, electrically this is an OK thing to do (take 24V from half the bank). But the problem is it leads to an imbalance in SoC between the two pairs of 24V (2x12V) batteries, which would eventually kill one pair of batteries. If the 24V load is very light compared to the 48V load, then it may take many cycles for the imbalance to be a problem, and before it gets to be so, you could swap the 24V load onto the other pair. Only you can say if this is an acceptable approach. Alternatively, an active battery balancer will keep shuffling the needed charge between the pairs.
On the otherhand, a DC-DC converter is a much cleaner hands off solution, as suggested above.
