I want to diagnose a faulty charging system in a European coach bus ('99 Neoplan). It has 2x 12V batteries in series and also two alternators. With the engine running I measure ~21V across the end poles (AFAIK should be ~28.8V). One (or both) of the alternators might be faulty. After taking the batteries out to charge them, I've measured the voltages one by one, and they were different. 11.8V and 10.5V.
I'm not sure if it's an alternator, a battery or a short somewhere else (starter motor maybe?). How would I start diagnosing a 2-battery, 2-alternator 24V system? How are the alternators usually wired?