The uppercut is a very awkward punch to throw, and yes with both hands the weight should be transferred to the front foot. When you use it make sure you bend your knees as you drop your arm down and come up with the punch. That will generate more power for you. If you have an uppercut bag (sideways hanging bag) its a good way to practice. Watch some fighters on tape throw it. Mike Tyson had short stubby arms so he threw his from the floor and somewhat unorthodox, but if your tall and lanky watch Diego Corrales he had a pretty decent uppercut for a tall guy.
The punch is somewhat similar to the hook in where your arm from your elbow to your glove stays locked and you generate the power from bending your knees, dropping your shoulder and rising straight up with the punch so that the power comes straight up the pole having the force of your body behind it.
Its kind of the same principle of all punches in that you want to generate the most amount of power in the shortest amount of distance to the target. I hope this all make sense to you?