There is more than one way to do a double kettlebell snatch.
Mike Mahler's technique (Mike is a Senior Certified Kettlebell Instructor) is similar to that used by Olympic weightlifters. Mike extends his body and drives his elbows back in order to pull the kettlebells closer once they are level with his head. This kind of double kettlebell snatch pull is an extremely effective upper back exercise on it’s own. When using this technique you should do lots of sets each consisting of low reps. Do not pull with your biceps, use your upper back and momentum instead. Also, drop the KBs back between your legs in between each rep. Once you get the hang of it, practice dipping underneath the KBs and locking out upon completion of the rep.
Following a heavy double snatch remember to ALWAYS lower the kettlebells to your chest before you drop them between your legs.
Brett Jones (another Senior RKC) performs his double kettlebell snatch in a classic "hard" style. His arms stay straight and rigid, with his hips driving hard without a second dip. If you want to work up to this, start by snatching the kettlebells higher and higher with every rep until they’re directly overhead.
With this exercise you will be dropping the kettlebells in one fluid motion and the force of momentum (or ballistic force) will be very high, so you should watch your knees and start out with a lighter weight than you may be used to in order to avoid hurting yourself.
Do not try this one-step version when you go to heavier kettlebells, instead start out with the two-step, pause at the chest method.