My HWH has four levelers. It is not possible to lift just one. You must lift either the front or rear.
You can raise front, back or sides separately.
I believe this is to prevent twisting of some frames.
My rig has a drive line parking brake and kick down levelers. If I raise the rear to much and get the weight off the rear it will roll in the direction of the grade. Of course, straight acting jacks will not under normal conditions do this.
By setting the front jacks first and then the rear, the rig is more stable and does not lift rig off the ground.