Unless he is stronger than you or thinks stabbing your arms is in the game plan or just starts waving the knife back and forth really fast, or hiding it from you while he grabs with the other hand and hides it again after stabbing your arms. To me people look like they are fumbling around when they try to grab a knife with both hands.
The modern Krav stuff I was taught is basically Karate blocks (360 blocking without the wrist snap so it is weaker) and fast counter punching (because people can stab you while they are knocked out). Holding the knifing hand and trying to wrestle is just more time you can get jacked up. People drop shit if you karate block their forearm hard enough. They lose interest if you punch them in the face (or knock them out with powerful, well trained striking and weight training).
Someone stronger than you with a knife wants to kill you and you don't have the option of running away or deescalating the situation you are almost certainly going to die. Furthermore, real knife attacks aren't from a distance where they show you the knife and start fencing. Generally you won't even see the knife until it enters your body once or twice. The thing is they don't stop with one stab and leave, they will keep stabbing you until you stop breathing.
At that point it's time to grab his wrist with both your hands, circle to the side like you're in a Russian tie, and every time he switches hands you have to switch arms. Do everything you can to make him drop the knife when it's not facing your body (biting, headbutts, groin shots, knees, punches if you have the perfect angle). If you're really lucky you might get the angle to run away. You still might receive some cuts in the process. If you have the perfect chance the karate block and counter then go for it, but if he doesn't get KO'ed then he's going to close the distance on you and then you're in real trouble. I'm not even saying my suggested defense is guaranteed to work. When someone has a weapon there are no guarantees, and you are inherently at a disadvantage regardless of how well you're trained.
Here are some videos that describe my thoughts better:
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On the off chance that you actually get the see the knife before you get attacked then if he's really close control the knife hand and strike with your free hand. If he's far away then strike hard, fast, and dirty. But always remember where the knife is, because on of his stabs is worth 10 of your punches.