Takwis:
Close, RBF. In the NG, the left one is labeled "MAIN ELECT" and the right one is "AUTO PILOT". MAIN ELECT "deactivates stabilizer trim switch operation" and AUTO PILOT "deactivates autopilot stabilizer trim operation". A few pages later, "The STAB TRIM MAIN ELECT cutout switch and the STAB TRIM AUTOPILOT cutout switch, located on the control stand, are provided to allow the autopilot or [my emphasis] main electric trim inputs to be disconnected from the stabilizer trim motor." They are wired in parallel.
I know what you mean, but electrically, they are independent circuits up to the NG. If parallel, either would force an on state.
Coming from the NG, the First Officer may well have tried cuting out just the autopilot trim, but there is no way for us to confirm it, because, in the MAX, the two switches are now wired in series. If one is off, all the electric trim is off. PRI stands for PRIMARY and B/U for "Backup". Both have the notation "deactivates main electric and autopilot trim operation".
And presumably, with the main circuit, MCAS.
The Control Column switches are overridden by MCAS, but the MAX Flight Reference Manual does not mention that.
Are you sure?