One single 172w drift will not perform good at all at 12v. The optimal range for ONE 172w drift pelt is between 18v and 20v (I used to run one).
The clamping pressure is what helps determine the amps it can draw.
For the best performance, an ampmeter and a variable output PSU should be used. Hook the rig up and put the comp under load. Start the pelt at 17v and watch the ampmeter and your load temps. SLowly start upping the voltage until you see that there aren't anymore amps going to the pelt and/or you aren't able to cool the processor down anymore.
I see where you are coming from. You are just running two pelts at under their performance rating as to keep the hot side cooler. That's kinda cheating, but gets the job done. I still say shoot for the moon on it though.
With a better performing cooling system for the pelts, you should be able to run them both separately to insure each pelt gets its own fair share of amps.