How does 2 stage furnace work?

Since there is only one heat exchanger and one orifice size on the burners does the gas pressure from the valve have 2 settings?
How is the temp rise maintained across the Hx with a 40% difference in btu output?
Could one manually convert a single stage furnace by reducing the gas pressure by turning down the valve to lower the firing rate?
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I can only imagine this backyard abortion project over a case of beer. Let us know how it turns out. On second thought, never mind. I'll probably be reading about it in the obituaries. Bubba
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