Some people are just better jumpers off of 2 feet than 1. Dominique Wilkins was one of the best basketball leapers of all time and he almost always left from a two foot takeoff. Having said that, to improve your single leg takeoffs, I'd recommend doing more unilateral leg exercises like lunges and stepups. Technically, I think you'd probably want to learn to be more comfortable in using the horizontal speed of an approach run to help you get vertical at takeoff. Many people lack the eccentric strength to takeoff efficiently while running. Also, be sure to lower your hips slightly on your second to last step prior to takeoff so that you are low enough to get an upward push with your takeoff leg.