When you don't have someone to tail the halyard (or two halyards) while you climb the mast, the Prusik knot is a solution.
Simply make a loop of a piece of small diameter (3/8") braided line and then wrap the loop around any taught, standing halyard feeding it back through itself twice, as shown.
Attach the tail of this loop to your harness and pull the knot tight.
Now you can use your hand to slide the knot up the halyard (in steps) as you climb, or as you are hoisted on another halyard.
If you happen to fall, the Prusik knot will jam, like a rolling hitch and stop you from falling.
On the way down you will need to slide the knot down the halyard with your hand again to prevent it from jamming.
I have used this as a safety backup when climbing my mast steps alone.
I even use it as a second backup when I have someone tailing a halyard as I climb.
Try it for yourself and make your own assessment and, of course, use at your own risk.
In a push, you could probably use two of these as a climber, sliding one up, and while standing in the other., but I think it will take some effort to release the knot after each step.
There is lots more on the prusik knot here
Howard [ Just Imagine ] 15-Sep-2008