Hedz personally I'd rather all dogs did at least a few double, especially snake heads. Found that cues that won't do it alone can be good o enter a pup just before he does it alone as they get the confidence or shown the should grab the fox before doing it alone.
I've found that some dogs, again especially the more traditional type will never grab a fox alone til they've be shown its allowed as there's no natural enemity and the scent puts them off so they but think its a game, then they grab it playing and it bite them they don't know what to do because if they were to grab (playing) an a