Choosing your two turbos for an STM build
The STM does not lock you into one turbo combo. That is the whole point. It is designed so you adapt whatever pairing your horsepower goal calls for. Here is how to think about it.
Primary: sized for response
The primary turbo handles low load and low RPM. You want it small enough to spool fast and give you that instant hit off idle and at part throttle. This is the turbo doing the work when you are cruising or launching.
Secondary: sized for your power goal
The secondary is where your horsepower target lives. Pick it for the top-end number you are chasing. Because the STM keeps it primed and brings it on smoothly, you do not pay the usual lag penalty for going big.
Model it before you buy
Before committing to a combo, model your boost targets with a proper turbo MAP calculation so you know each turbo is landing in its efficient range. Garrett publishes a solid walkthrough for turbo selection and the math behind it.