Cactus1 = The Truth Cactus
Cactus2 = The Fraud/Lier Cactus
"Which door would the other cactus say is the safe door?"
Cactus1 will point to the Doom Door as that is what Cactus2 would point to.
Cactus2 would also point to the Doom Door because he will HAVE to LIE since he doesn't speak the truth even if Cactus1 says the truth and therefore the only door will be left is the door that isn't chosen by any of those cactus and that leaves us with... the Safe door.
Actually to clarify it?
You only need to ask just one Cactus the question, "Which Path would the other Cactus say leads to Doom."
The Path shown is the Doom Path.
BTW this Riddle first came up on "Doctor Who" a BBC TV Series some 30 odd years ago. and was one of many enigmas raised in the Series.
Thanks for bringing it up again.