Climbing harness: clip two carabiners to avoid cross-loading
For reasons of time-saving and avoiding retie figure 8 knots all the time, can I clip the figure 8 knot via two carabiners each clipped to either loop of the harness? This way there will be no cross loading of the carabiner. Clipping is way faster than tying figure 8 knots.
The carabiners are double-locking ones.
UPDATE:
First, thanks to all of you. You provided such an amazing info! Can't stop clicking upvotes. I see there is missing some clarification about why I need all that. So I'm putting it here.
Soon I with my kids will go to a hiking trip involving several sections of scrambling through a canyon. The youngest is 4 y.o. I have harnesses for the elder kids and full-body kid's harness (Black Diamond Momentum). Since we will have several such sections of climbing and regular hiking between them I thought that clipping and unclipping carabiners would be much faster (mind the fact that I will safeguard all three kids). The scrambling sections themselves are not very long, so they will not take much time to pass. There are anchors on top of each section, so my plan was that I climb, clip the rope, then probably go down and safeguard my kids one-by-one (there will be more adults).
There will be no hard falls, since I'll watch them and will keep the rope tight through my belaying device. Having that convenience of fast clip/unclip is hugely important and also safer. Because if you stuck there after sunset - it's no joke.
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3 answers
Lots of people will double up twist-lock carabiners for various reasons, and it is considered an acceptable method of protection by many, although in my opinion it is very unnecessary (related: Is it ever necessary to double up locking carabiners?). They do not however bypass the belay loop as you are describing by clipping a biner into both the seat and belt loops independently. Doing so could eventually cause one biner to open the gate on the other, especially if using twist locks.
The best method I know of for clipping a climber in with a carabiner is to tie a tight knot onto an auto-locking, gatekeeper-style, pear-shaped carabiner such as a Black Diamond Magnetron Gridlock carabiner, or a Grivel Clepsydra Double Gate biner:
Grivel Clepsydra Double Gate secured to rope by wrapping the rope tight around the biner twice, then tying off with a double overhand (a round turn and double overhand noose - the rope will break before this knot comes undone), but you can also tie a tight figure 8 on a bight, a water bowline, etc.
This is the simplest, fool proof method for tying into a carabiner while top-roping. Cross loading is eliminated, and your transition between climbers is a lot simpler and quicker.
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I don't think it is great practice, but lots of people top rope off of a single biner attached to the belay loop. The belay loop is plenty strong for this purpose, but the carabiner adds an additional single point of failure into the system. Two locking carabiners that are opposite and opposed would provide redundancy in the case of failure. Using the belay loop is generally more desirable as it reduces the change of cross loading as there are only two points of contact (the rope and the belay loop) as opposed to using the harness where there are three points of contact (rope, waist loop, crotch loop).
Some climbing gyms may allow this, others will most definitely not allow this. I believe, but cannot find it on line, that some gyms actually require you to clip into pre-tied knots. The idea being the chance of the carabiner failing is lower than the chance of the knot being tied incorrectly.
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Based on your edit that adds additional context your plan seems reasonable. In addition to the carabiners ShemSeger recommends you might consider a captive eye carabiner (any rated biner is fine. I'm just using these DMM biners as an example). These are often used in more industrial settings so they're often found in steel. Alternatively you can get a DMM Belay Master 2 carabiner. These are designed to ensure the biner is locked properly and prevent crossloading Finally, I agree with StrongBad that you should use the belay loop not the tie in points.
Original answer
This heavily depends on context and exactly what you mean by "either loop of the harness."
First things first. The only loops that are load bearing on a standard climbing seat harness are located in the crotch area. These are known as the tie in points. Any other loop on the harness is intended to hold gear and isn't strong enough to hold your weight.
Assuming you're talking about load bearing attachment points on the harness carabiners are only safe from cross loading in one very specific scenario. The carabiners need to constantly loaded so they maintain the appropriate orientation at all times and don't allow the rope/carabiner to move/rotate. The only way to really ensure this is to be climbing in a firm (constant tension on the rope) top rope situation. This is why the climbing towers you see at fairs with automatic belays are safe (among other reasons) even though they're using carabiners. These situations also benefit from the improved speed you mentioned. Constant tension on the carabiner is also why people aren't really worried about cross loading carabiners mid-rappel.
In any other climbing situation/scenario the modest additional speed gain and significantly decreased safety margin aren't justified. I've climbed with many different people both outside and in gyms. Unless you have a group of people working a single short easy top roped climb the climbing time is going to be much greater than the tying in time. It simply doesn't take much time to tie and untie a knot. Occasionally I've taken many falls on a climb and had to untie a rock hard figure 8 knot when my forearms and fingers are pumped. Even then the entire amount of time to untie was less than 5 minutes, and the time it takes to tie a figure 8 should be less than 2 minutes including dressing the knot. You shouldn't feel so pressured to avoid an worst case scenario of 7 minutes of work that you're willing to risk a ground fall from height.
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