db16
you obviously turn one pack ON, then APU OFF and then pack 2; that is what I was taught on the 737 groundschool!
Are you talking about after start or for take-off?
At idle power after start you would probably have all 3 bleeds selected on (depending on aircraft type) but the system should be designed to cope with that.
Surely for a bleeds off take-off it would be eng bleed 1 & eng bleed 2 as with the packs off you would be unpressuised so there would be no point in having the APU bleed on in the first place