"Bob Gardner" wrote in
:

If the switches are in parallel and one of them develops an internal short
circuit, you will still have trim runway; if they are in series, both must
be closed simultaneously. Sounds like a dumb idea to me, but check with a
mechanic or radio shop for a knowledgeable answer...I'm not sure that I
would expect the Chief Pilot to be the expert on electrical questions.

Bob Gardner

Although it's been a while since I flew a 172SP, IIRC, they're just series
switches. I don't even think it is the "one controls the clutch, the other
controls the motor" theory that a few people have responded with.

On the Arrows and Bo's in our club, the Electric Trim is a single toggle on
the yoke, and there is a cutoff switch on the panel to deal with runaway
trim...

It's not unusual for one to push the trim switch up or down and nothing
happens, but I can't recall a situation where the button got stuck in a
movement position. Still, I can see where there might be advantages to either
approach to this safety feature.