That's what someone else was talking about, ATN topclimber.
My solution: go to REI or some other shop that knows climbing. Buy 2 ascenders and a rockclimbing harness and essentially replicate the idea of the topclimber. You will get much better quality (rockclimbers know their gear) with better safety (you can hang upside down in a properly fitted rockclimbing harness without falling out; don't try this in a bosun chair!), for a fraction of the price.