Air Flow Theory Question with Multiple Stacks Involved

I have a degree in science, but not engineering. The stack effect, I believe it's referred to, is easy to understand when there is a single stack involved; warmer air from inside a vessel moves up and out the stack as cooler air is drawn in the inlets of the vessel. Increasing the hight of the stack increases air flow out of the stack. But what happens when a second SHORTER stack is added to the top of the vessel. Will the shorter stack always act as an outlet or will it act as an inlet because it's shorter than another stack. Will it compete with the taller stack?

My scenario doesn't involve a building with floors, walls, or any means of artificially heating or cooling of interior air. Imagine the vessel is an empty tank and the air temperature inside is the vessel is always higher than ambient temps. Also assume that when the second stack is added, an equal area of inlet vents are added to the lower part of the vessel. Thanks.