A lot of beginner aid climbers want to add a “backup” carabiner to the top hole in their ascender. It’s not wrong, but usually not optimal. (Note: use an oval carabiner)
I get it. The first few times you use ascenders it’s pretty scary, and you probably want every possible psychological safety. If that biner on top can lessen the chance of the ascender coming off by 0.1%, that is still a good thing to have, the reasoning goes.
Here’s the thing. You only really may need this backup biner if the rope is stretched tight and you can’t tie backup knots, and/or if you’re cleaning a roof, and/or if the rope is running at an angle greater than about 45 degrees off to either side. In other words, some oddball rope angles not encountered on a normal pitch of cleaning.
And, if you’re worried about the ascender popping off, remember this. If it does, you have your second ascender, and probably the rope running to your Grigri as a third point of contact, as well as being tied in to the end, a fourth point of contact. And if you’re really being cautious, you’re tying backup knots, which is a fifth point of contact with the rope.
Try this - set up a tensioned horizontal fixed line, clip an ascender to it, and try to torque it in every crazy way you can think of to get it to pop off. It’s darn hard to do it! Not to say it can’t happen, but it may be more secure than you think. And always be sure the ascender fully clicks closed onto the rope every time you put it on, especially if the rope is under tension in a weird direction.
Plus, here’s the main reason why you usually don’t want this biner in the hole. If you have to clean a slightly traversing pitch, you may need to remove the top ascender and leapfrog it past the piece of gear you want to clean. To do this, you need to remove and re-attach that carabiner every time, which is a rather major time waster if you have a more than a few pieces to clean.
So, it’s best practice, most of the time, to not clip that carabiner through the top hole. If you do, it’s probably best if you use an oval carabiner. Also, be sure you clip it correctly around the rope, and not in the completely incorrect position shown below.
Be sure you don’t clip the hole like this!
If you’re ascending a severely traversing pitch, it’s probably a better plan to clip the locker that’s connecting your daisy to the ascender to the rope with spare carabiner, like this:
Doing this will help keep the ascender more in proper line with the rope. (Aider and locking biner not shown)