Q,
Are you sure that your extract from the AMM is correct? My Vol 2 Systems manuals for the 3/4/500 and 700 all state that “AUTO - When operating one pack from APU (both engine bleed air switches OFF), regulates to high flow.” I can’t believe that you would want to drive the APU so hard when it is your only source of air. I may just try it on the ground tomorrow under engineering supervison!
Re both engine bleeds OFF scenarios. The various smoke/fumes drills do not call for any change in engine bleed position until you come to smoke/fumes removal, which then asks you to verify that both engine bleeds are ON. The best way to deal with a dual bleed problem is to switch the APU bleed OFF, not either of the engine bleeds. Even wing-body overheat drills only ask for the bleed on the affected side to be switched OFF, not both.
The only time I ever switch them both off is not for a non-normal, but for a bleeds off take-off or landing. Bleeds off take-offs are relatively common (maybe 1 in every 100 flights depending on your destinations and loads), but I have only ever had to do one bleeds off landing in 10 years on type. That was on an 18.5K engined, –500 on a flap 40, Cat IIIa, autoland in icing conditions. If you go through the perf books for that config you will find that you are so weight limited for the go-around climb gradient that you can barely shoot the approach with enough fuel for a diversion even if you have only a modest pax load – a very uncomfortable situation!
Regards
S & L