Since turning the heater knob alters the mix of air I think it is fair to assume that one flap controls the passage of "cold" air and the other the passage of hot air that has passed over the heater element.
As the visible flap controlling hot air flow is opened the other flap must be closing in such a way that a portion of the air entering the system passes over the heater element. When the heater flap is fully open the other flap should be fully closed in such a way that all the air passes over the heater element.

I put the "cold" in inverted commas because, as Mark has clearly shown, the air entering the system is above the ambiental temperature.

Peter

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