Try to position the strings so they aren't too close together on your index finger. Hit double or nothing with the string closer to the base of your throw-hand index finger, then when you go for the double or nothing brother you want to hit the string closer to the tip of your finger. That should give you enough space to land on the correct string.

That's the YoYoJam Dark Magic 2. If your yo-yo isn't spinning long enough try practicing your throw and make sure you are properly maintaining the bearing.