Alchad
I am quoting from memory, but the explanation somewhere on this forum is:
1. On the NG there are only two inputs for electrical trim, the
autopilot flight control computers and the yokes. Each has their own switch, wired in parallel.
2. On the MAX there are three inputs, if you add MCAS. Since adding a third switch is complicated, they wired the switches in series. This provides an extra level of inhibition of runaway trim, since one of the two could theoretically get stuck in the on position (due to short circuits).
I hope this is correct and helpful. The implications of these changes are beyond my scope.
Edit: I posted simultaneously with
infrequentflyer789 whose comment overlaps with and supports my recollection.