Do you mean 2 prusik's for the climb and ATC or Figure 8 to rappel down? I guess you could rig the ATC/8 for the climb up but I dont think it's a replacement for an ABD, I think you still need 2 prusiks. I have used a ropeman 2 in place of my hand ascender and I had the lifeguard but returned it because the grigri rappel was a lot smoother.
Also while you can get away with only a rope the length that you climb, its nice to have the option of doing a 2 rope rappel with an ATC because it makes the rope retrieval a lot easier. You just wrap the rope around the tree, but you need a good sized branch. I carry one 60 foot rope, rope can be bulky but I'd rather carry 20 more feet just in case, the weight difference is negligible.
Also I posted in your other thread where you asked for the interest and I think it's worth mentioning here. Please be extremely careful with the assisted braking devices (ABD) which accounts for your Grigri or Lifeguard they are not designed to be used without your brake hand on the tag end of the rope. Especially while rappelling. Be careful and follow the manufacturer's instructions.
Also consider keeping 2 attachment points at all times, I keep a daisy chain for this reason, it connects the hand ascender to the grigri or harness, which will shorten your climbing strok(longer daisy chain might change that, hmmm) but keeps you connected to that rope in 2 places.
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