If you have two GFCIs connected in series, which would trip first or both?
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.