Ya the other is in the valley, pressure side goes straight to the other turbo. it runs much cooler than a single turbo set up. One feeds the other, much quicker spool up, WAY cooler EGT's, lower drive pressure (the pressure on the exhaust side, high drive pressure is the cause of lots of different issues) and makes twice the boost. the stock VGT in the valley makes boost right off idle and the big atmospheric (75mm) starts to spool at about 15 psi. Together they make about 60psi.
I can pull 10 grand up a 6% grade on the hottest tune (more fuel/timing=heat) at about 125km/he with the truck on cruise in OD and it won't go above 850-900 degrees. Before this truck had twins it would spike 1500 degrees in 20 seconds trying that. I have a video to prove it too. Running empty I can't make my EGT gauge go passed 1250 if I tried, which is pretty cool considering the tuning in running is pretty wild. And it will still get about 18mpg empty, that's with 35's too.
The main purpose of a set-up like this is for cooler operation, reliability and more HP. The ONLY downfalls are more of a clutter under the hood, oil temps are a little higher at times cuz your lubing two turbos instead of one and I have zero emissions system in place so if they change the laws here I'm screwed LOL. Other than that it's been Bullet proof, 130k miles on it now running big boost with no issues(lots of supporting mods though) It's a ton of fun on the street, not much will touch it except some sports cars and it tows like a dream.