Doesn’t having two coils plumbed in series render the second coil ineffective?

No

 

Configuring the coils in series is much better from an equipment and temperature controlling perspective, starting with the fact you won't require a second pump or controller.  

 

In fact, the pump moves the chill source water at a much higher velocity than you are envisioning.  For this very reason, the temperature difference between the water entering the coil and exiting the coil is only roughly 3-10 degrees apart depending on the temperature of the beer.  So there isn't a drastic drop from say a 65 degree set point for the beer and the 35-degree chill source water.  We designed the system this way to keep a consistent coil temperature and prevent the system from overshooting, which can happen often with jacketed fermenters because of the overabundance of surface area contact between the chill source and the beer.  In glycol applications, icing can even occur. 

 

As a result, adding a second coil adds more surface area contact with the beer inside the fermenter for more advanced cellaring practices like lagering and crash cooling. Yet, with two coils in series and 35-degree chill water, users won't overshoot their desired set temps like they could with two independent loops.  Lastly, the added trouble of syncing two controllers, each with their own dedicated thermowells isn't ideal either.