Once you can axle stall, 50-50's arent much harder. It involves going up as if you were going to do an axle stall, but at an angle and with more speed. If you follow all the stages of an axle stall, you should get both trucks on the coping but you will be sliding along the coping. If you find yourself falling off, just lean slightly into the coping when you enter the 50-50.