I explained it in detail long time ago. The rear vents are just an extention from the cooling box at front. To have decent flow in rear you need to have full blowing motor in front as its same cooling box sending air in different directions.. means more CFM in cooling box .. more flow in vents.. if you lower through /speed in front you will feel lower air pressure in rear.
In case of dual climate control , it's same situation.. dual just provide diffrent air temperatures to driver and passenger.
In high end luxury cars have three zone.. means there are individual controls in the rear vents. Rear passenger can change temperature as well as flow / speed. It works independently.
In Civic , the rear vents are just extention. OK simple example. You have garden hose to water the plants.. you gave two extensions taking water in diffrent directions. Now add another extention.. you will need more water volume and pressure at source to have better water dripping out.
I have rear vents I'm my car and I have dual zone unit.. if rear passenger need more air.. I have to increase the speed to fan.
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