When wall-running, you will only stick to a wall as long as your centre of mass (COM) is moving toward or away from it; as soon as you stop, you drop. When using a one-step technique, the leg on the wall first has to absorb the impact as the COM moves toward the wall, then it must push the body away.
The vertical movement first comes from the jump from the floor. The leg makes contact with the ground, absorbs the impact, then pushes the body up from the floor. The step on the wall is the same thing; as the leg first makes contact with the wall, it starts absorbing the impact at an almost horizontal angle. Then, as the COM rises in relation to the foot on the wall, the angle gets steeper so that as the leg continues to absorb the movement toward the wall and then push away from it, the force is directed in an ever more downward dirction (making you go up!). The trick is to time your effort to generate the maximum explosive force from the leg on the wall at the steepest angle that the traction between your foot and the wall will allow.
The difference with a two-step technique is that the first leg on the wall absorbs the impact as usual, but then the other leg contacts the wall and performs the away and upward movement. This technique can be used to good effect by those with one leg significantly more powerful than the other (which is a significant percentage of people) as it allows the favoured leg to perform both major explosive movements. It also has the advantage of allowing the push from the wall to be performed from a slightly higher position. The drawback is that more effort is spent messing about with feet on the wall intead of getting up the wall!
Overall I have found that the best one to use varies with the individual, but I would recommend that both techniques are perfected so that you can make the best decision as to which one really works best for you.
(By the way, I have made a point of travelling to meet several different people who claim 14 and 15foot wall-runs, and not a single one has ever actually been above the 12foot mark. Not that matters in the slightest, but if you want to know how high you can actually go, please measure accurately.)