Double-Crux is a technique for addressing complex disagreements by systematically uncovering the cruxes upon which the disagreement hinges. A crux for an individual is any fact that if they believed differently about it, they would change their conclusion in the overall disagreement. A double-crux is a crux for both parties. Perhaps we disagree on whether swimming is a lake is safe. A crux for each of us is the presence of crocodiles in water: I believe there aren't, you believe there are. Either of us would change our mind about the safety if we were persuaded about this crux.
A version of the technique is described in Double Crux – A Strategy for Resolving Disagreement written by (then) CFAR instructor, Duncan_Sabien. The Center for Applied Rationality (CFAR) originated the technique. Eli Tyre, another CFAR instructor who has spent a lot of time developing the technique, more recently shared The Basic Double Crux pattern.