Except it’s not just a mechanic, it’s an unique identity of Vulpacea troops.
Dual spell is the main one, while Fox Cunning with 25% start is the secondary. If any troops can have it, then it might make Vulpacea troop less special.
Most of the time, troops with OR in their spell name was meant to work exactly like that. You have one guarantee effect, and secondary one that you wish it will work in your favor. If we’re meant to be able to choose, then might as well make a new troop for that, as unique identity will be broken.
That doesn’t mean rework is not possible. There is a lot of Kingdom Rework on in the future now (with the pass), so maybe one of the OR troops will get an improvement. But still, it’s probably in a way of making each options stronger, or discard the rest, keeping only the gaurantee one that fit the best for its identity.
I can see non-Vulpacea Wargare troops get the duel spell mechanic without the Fox Cunning trait though, and the lore can be stated as being distantly relate to their main kingdom. Maugrim Woods troop and Wild Plains’ Gnolls are both a good candidate, but any kingdoms can work too with fitting theme. Non-Wargare troops are probably out of luck to get it at the moment.