Dual Trim Switches?

After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")