Jumping with two feet has the potential to get you higher off the ground than jumping off one -- if you're an elite gymnast. Gymnasts can generate great momentum by, for example, sprinting and doing several back handsprings as they approach the jump. Successively whipping their bodies forward increases the amount of force they apply when they finally launch upward off both feet.
The extra force gymnasts generate is significant. Elite gymnasts can cross a high-jump bar at heights of 9 to 10 feet, according to the book "Sport Mechanics for Coaches," by Brendan Burkett. In contrast, the world record for the high jump was just over 8 feet as of 2010, according to Burkett. The disparity exists because the rules governing high-jump competitions forbid two-foot takeoffs, as well as handsprings or other gymnastic techniques for generating additional force.
If you can't flip and orient your body in just the right way, as elite gymnasts can, you're more likely to hurt yourself than to jump high. So for most people, the best option for jumping high depends on whether they're moving or not. If you're jumping from a stationary position, two feet can get you higher off the ground than one. If you can take a short step before jumping, you might be able to get a little bit higher.
Benefits of Two-Foot Jumps
Jumping with two feet has some extra potential benefits. Pushing off with two feet makes it easy to control your body while in the air and can help you land safely. It's also easier to perform quick jumps in succession. For example, basketball players often use the two-foot jump to grab rebounds or block shots.
Jumping While Moving
If you're running, you'll jump higher if you launch off just one foot. For example, basketball players often leap off one foot to dunk the ball or shoot lay-ups. Consult a certified fitness expert or an experienced basketball coach to show you how to jump off one foot, but the basic technique is as follows: Start with a fast sprint and then stamp hard on your jumping foot. As you stamp, briskly raise your other leg and the arm on that side to help increase your momentum. Note that you will have less control of your descent and landing than you would with a two-foot jump, so be sure the area is clear before practicing your one-foot jump.