I'm SLF but have enough of a technical background to add something here. My understanding is that the control column stab trim switches have two series contacts specifically to minimise the probability of single switch contact fault causing a stabilizer runaway. The first contact operates an arming relay regardless of the trim direction and the second contact provides the trim direction control. Signals from both contacts are required to be present to operate the stabilizer electrical trim. (the schematic diagram in #3882 shows this nicely). The arrangement is a sound and common principle which I think was derived from bitter experience.
As for the Stab Trim Cutout switches on the Max, if the schematic is correct then selecting the Main cutout switch to CUTOUT stops all electrical trim (including MCAS) by, amongst other things, de-energising the main trim control relay which cuts power to the stab actuator. The Backup switch is in series and since a series switch has never been needed on any previous 737 seems to be superfluous so I'd agree that Boeing put it there because there have always been two switches.
With both switches selected to CUTOUT as they seem to have been on ET302 then re-selecting the Backup switch to NORMAL would have no effect on anything. If the Main switch is then selected to NORMAL then both main electrical trim and MCAS would be active. If only the cutout switch nomenclature and not the new functionality on the Max was communicated by Boeing then the cockpit of ET302 must have been a bad place to be learning about it.