Two GFCIs in series, which would trip?
All of them
And good luck resetting them.
I mean, it doesn't do any harm, (except in the sense that security at the expense of usability is at the expense of security, i.e. someone tears it out because its annoyances drive them nuts).    It just doesn't provide any benefit, and as such, is just a silly thing to do.

ref
Every device is seeing the ground fault, so every device would trip.  One might imagine a race condition where one device sees the ground fault first and physically severs power before the others are able to detect it.  However there is a mechanical delay between detection and the hammer physically tripping, and that delay is considerable in the time scale of detection.  As such, all detect the ground fault before any can actually interrupt. The physical energy to trip is pre-loaded in the mechanism (loaded by you pressing the &quot;reset&quot; button), so it doesn't need power to trip.