Picture this: increase inverse voltage across your 5408's in series. It builds until it reaches brekdown voltage on one. Current would flow and destroy one, but the second, running at less than breakdown PIV does not allow it to pass. One diode runs at breakdown PIV and the extra is taken across the next one.

Since Schottkey types were nor recommended for this service, I think they must short out in over-voltage failure, and fail in a domino falling scenario.
regards,
Douglas