I don't see a reason for them not to stack, but maybe I'm missing something.
You still need at least Karandras and a warlock as warlord(or say an autarch warlord to re-roll 1's and a warlock) and then both cast enhance and assault a unit in cover and use a stratagem.
That is a lot that needs to happen simultaneously, and it's only as powerful as the psytronome of Iyanden, (because it roughly doubles your number of attacks) albeit there is no mortal wounds suffered from it.
As far as I can tell the Exarch power and Karandras power does stack too right?
So the exarch could then trigger 3 rules at ones, that all grants an extra attack each.
However only one of those extra attacks get the better AP as I interpret it.
While on the subject scorpions, could the blazing star of vaul be fitted onto a scorpions claw?