In normal operation, the "BAT EPU" and "BAT/DCT" (or however the second switch is labelled) are treated as one switch, I press/engage/disengage both simultaneously. The BAT EPU controls all busses; the BAT/DCT only powers instrument panel and console switch lights without everything else. If I wanted to check switch position, altimeter etc prior to start, I would engage only that button.
Connecting an external electrical power source energizes all busses excepting the aircraft battery and generator once you engage the BAT EPU switch. The aircraft will run on the external electric power until disconnected/depleted. One can't engage the generator or battery if the external power relay is active.