Originally Posted by
Raskolnikov
One of the reasons your second pull is lacking is that you open your hips up too early. You lift your chest as soon as the bar comes off the ground, which means your hamstrings have already shortened by the time you initiate the second pull and they aren't really able to contribute as much as they should (similar to why lifting your chest out of the hole during the squat kills hip drive).
The first pull is all about positioning: you want to get the bar from the floor to the "power position" right around mid-thigh (varies from person to person) without much, if any, change in back angle. In other words, you want to stay over the bar as long as possible (bar directly beneath your shoulder blades). Think of the first pull as all knee extension without any hip extension, just like a halting deadlift -- then, after the bar has passed the knees, think "jump!"