Ian W,.....I meant to put !!! instead of ????..
Sure you can switch the CACs off, both of them, then you have no air coming in,but may already have a cabin full of smoke...
Point is,on the 787,the CAC intakes are much closer to each other,than with the separation achieved by using engine bleed supplied air.
So smoke entering the cabin via one intake,could also, be more likely to enter via the other,too...as would most certainly have been the case,in Vegas.