I have read about exhaust nozzle design and how it affects drafting and backpressure. My understanding is that the exhaust blast spreads into a cone shape above the nozzle, such that it seals the throat of the petticoat pipe. This creates a vacuum in the smokebox that draws combustion air into the firebed and through the tubes & flues, and then entrains the flue gas with the exhaust for ejection from the stack. Sort of like an airbrush draws paint through a straw. For a single stack locomotive, this seems like a fairly straightforward process. For locomotives with twin stacks, it seems to me that exhaust steam would need to be flowing through both stacks at the same time to prevent atmospheric pressure from killing the vacuum (and therefore reducing the draft) if one of the stacks was between exhaust pulses. Now, I am familiar with the designs of the UP Jabelmann twin stacks on FEF2/3 Northerns, 3900-class Challengers, and Big Boys. They employ an exhaust stand that allows exhaust steam from the front and rear cylinders to "communicate" with both nozzles and therefore both stacks. The C&O Allegheny, on the other hand, used what appear to be separate exhaust stands with no means of the rear engine exhausting through the front stack and vice versa for the front engine and the rear stack. See erection drawing attached and the dotted lines which show the front and rear exhaust stands, nozzles, and petticoat pipes. Note that the front stand collects exhaust steam from the front cylinders and the rear stand collects exhaust steam from the rear cylinders and there doesn't appear to be an exhaust connection between the front and rear stands.
Now, the question, did this work simply because the time between exhaust pulses was short enough to ensure that most of the time both stacks were sealed with exhaust steam?
