I have two lines of 25 waterwheels rotating opposite of each other, each wheel producing 320su (with soul sand water columns), each line producing 8000su, and both lines are at 20RPM. So when I combine them I'd expect to have the stressometer read out x/16,000su(x being the stress I'm already using), but what I have is x+8000/16,000su... so I'm not gaining any usable SU by adding the extra line. 

 I've tried remaking this exact setup on a smaller scale, and that works as expected with the exact same method of combining the SU(a combo of small gears and belts). Either something is bugged or I'm missing a very simple answer. I'll try rotating the waterwheels so they are both going the same direction later, but I figured this is worth reporting.