Dear Rainboe,
I respectfully disagree: you can have the left AC Transfer Bus get its power from the left IDG while the right AC Transfer Bus is powered by the APU Generator...
Now my question concerns external power mixed with an IDG (naturally not on the same side)
Regarding the first question: The battery bus can also be powered by the battery, right? But for this to happen the battery switch must be ON? (And of course TR3 must be unpowered)