Doesn't the Power Supply Assembly use a dedicated battery to prevent power interruptions during power source transfers?
The "fly by wire system" has dedicated batteries that keep the computers powered during short power interruptions. These "dedicated FBW batteries" are normally kept charged by battery chargers powered by the AC generators.
All AC bus-bars loose power with associated generator failure until the other (or, if running, the APU) generator restores AC power (ignoring the limited AC produced by the static inverter)
However, never in all my years understood why they are called AC generators, not alternators!!