"Adding the arrow" part was confusing for me. But that shouldn't matter much.

Placing two photomultipliers next to each other, one of them only goes off at a time per instance photon, never both at the same time which is what can be expected to happen if the photon was a wave (a wave would cause both to go off).

Might be a stupid question. But how/why would a wave cause both to go off?