This is a fantastic question.
There are some SKILLS that allow you to roll against the Target Number instead of using your standard stats. If that ever does roll 12 over, then that’s a critical success and you can mark double.
What I personally do in my games is that if the effect states it is a CRITICAL HIT when you roll certain dice outcome, then that is a double success marker on that skill. However, I do limit by saying that you cannot gain additional success in that Combat or that Scene if you have already filled that skill up, until you can rest and gain that new 1D.
When it comes to skills that do not roll against the Target Number AND do not have a critical condition built in, I still use the 666 rule AND I just compare the dice total to the established target number. If Crit, then double success.
I hope this gives you an idea.