You don't need us to tell you. You already have the answer. Certainly if you only need a rough guide.
Two 24V batteries each of 23 Ampere Hours and, in an emergency situation each feeding a separate buss (one AC and one DC. Not that this matters for the calculations). If the batteries are rated at 23AH and must last 30 minutes, then the average current draw on each buss will be???? Or do they not teach this simple stuff when they teach degree level engineering any more?
Anyway. Your fuel cell must supply 1.1 kilowatt to meet the above requirements for each buss. Then to be certified it must be rated at 1.5 times the requirement. So that means 1.6(ish) kW.
Therefore, if you intend to use a single fuel cell it must be 3.2 kW to supply emergency power for 30 minutes. As you have said in todays regulatory environment 30 minutes may no longer be sufficient.
You will also have to consider that if you remove the batteries. how are you going to start the APU from an unpowered airplane?