rMBP Trackpad slips when trying to scroll?
Normally, this means you just don't put both fingers at the same time. Try sticking your index and the other finger together and moving your arm or your wrist instead of trying to move the individual fingers. You'll get used to it.

The animated videos in the System Preferences -> Trackpad are a good example of the safest way to do the gesture. Once you get the hang of them, do them like you want, but it's a good starting point.