The small turbo COMPOUNDS the boost from the big turbo. In a nutshell, the big turbo is the small turbos intake. The small turbo isn't spinning any faster than it was at 8-9 psi, but the intake charge is 3-4 psi, versus atmospheric pressure.
Exhaust gasses first go through the smaller turbo, which starts it spooling. The gasses are then plumbed to the big turbo, then out the exhaust of the truck. With the small turbo spooling, it's introducing manifold pressure(boost), and in return, more exhaust gasses(drive pressure), to not only turn the small one, but to also spin the big one. At no point the large turbo going to spin as fast as the small one, nor spool as fast either, they are not designed for that.
The little one gets stuff moving, the big one makes the power for the big end. Compound turbocharging at it's absolute simplest form. Obviously other variables exist, internal and external gates, housing sizes, wheel trim, etc.