Hello,
Now that my DC-DC is working again, I have a follow-up question: do I need to use the same battery settings in the DC-DC and MPPT configurations, or does it not matter?
Yes. It matters. Your battery charge settings are based on your battery type. Not the charger.
And I assume that with the same battery settings, the two chargers should report the same state—bulk, float or absorption—at the same time?
Not necessarily, the decision to change is often based on the voltage and current at the charger. The devices may have different wiring paths with different voltage drops between the battery and the charger and not share the current equally, also slight tolerance variations, so often you find one charger going from bulk to absorption or absorption to float before the other. This can be avoided if you have a SmartShunt / BMV712 and then either set up a VE Smart Network or DVCC on a GX device, in which case the chargers may well use a common voltage and current reading if they can communicate in this manner.