Its early in the morning, so this is with some hesitancy, but hey, I've
already embarrased myself.
David remarked, concerning the requirement that all field lines starting on
plate A must terminate on plate B.
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Notice that this requirement of no net excess electric flux external to each
of the capacitors is equivalent to a requirement of a vanishing field in the
region between plates B and C.
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A very minor quibble, they are not precisely equivalent; as an odd field
line or two may leave plate A and terminate on plate B by first going around
plate B and entering that region between plates B and C.
For ideal infinite parallel plate capacitors, they are precisely equivalent
(as David, quite quickly and correctly, mentioned in pointing out one of my
errors). And I presume that in most reasonable geometries (not extreme
geometries) they will be very very close to being equivalent, as almost all
field lines would be in the region between plate A and B.