When the EXTRA SUPPLIES TRIP is pressed, that would automatically isolate all external supplies and would automatically connect the No.2 Alternator to the synchronising busbar to ensure electrical power isn't interrupted.
With no external supplies connected, the real aircraft checklist then calls for you to synchronise all alternators, check load sharing, and then isolate No.1, 3, and 4. You then need to reconnect the AAPP by pressing the AAPP ON button (which is the step skipped in the tutorial video) and that will automatically connect the AAPP Alternator and isolate No.2.
From the 'After start checks' section of the real aircraft checklist:
AAPP . . . . On synch bar
a. Within limits
b. ON button pressed (1 second max)
c. Check indications
Extra supplies trip . . . . Press, No 2 alternator on synch bar
External supplies . . . . Removed
a. Palouate
b. External 200V
c. External 28V
Alternators . . . . Load sharing checked. No 2 on synch bar
a. Synchronise all alternators
b. Check load sharing, kW /kVAR
c. Isolate No 1, 3 and 4
AAPP . . . . On synch bar
a. Within limits
b. ON button pressed (1 second max)
c. Check indications
Mark - Just Flight