In 37 years I don't think I've ever TR'd with two lockers, just two opposed non-locking biners or draws. I suppose if you're paranoid or engaged in group commerce, but seems like overkill to me no matter how you orient them (which in itself also seems like a silly subject).

I also agree that D-shaped biners can pinch the rope thus nullifying the effect of a greater bend radius which is one good reason to add a second biner.

In the thread that I linked to it was suggested that using two lockers with gates to the same side is safer than the same lockers o-n-o.

However what you or I climb on was not the question. In the thread that I linked to it was suggested that using two lockers with gates to the same side is safer than the same lockers o-n-o. Since this is the beginners forum I'd like to have that statement verified or debunked.

I know the conventional wisdom that the pinch adds more wear and tear to a rope, but I've never heard that it amounts to reducing the rope-bearing radius to one biner.

I with everybody who thinks arguing over lockers, reversed and opposed or otherwise, vs. reversed and opposed non-lockers is a waste of time.

BTW, what is "O n O?"

it is suggested that opposite and opposed loockers at a top rope anchor is somehow less safe than double lockers oriented.

If you're using 2 lockers it doesn't matter how you orient them.

The primary reason we still use two 'biners at the top rope anchor is to increase the bend radius of the rope and reduce the internal friction from falling or lowering over a tight radius (bend radius is ideally at least 4x rope diameter).

At the risk of giving pedro another opportunity to start an endless flame war, I will emphatically disagree with this statement.

Most lockers are screw locks. Screw lock 'biners, if oriented vertically, should always have the gate opening facing downward so that the locking sleeve is less likely to unscrew.

And that's a great mnemonic for teaching nubies: screw it down so you don't screw up.

I dunno. Sounds like some really perverted sexual position.

opposite and opposed

However an experienced rescuer, rigger, guide and whatever else you claim to be really should know that adding karabiners increases the friction at the top anchor.

You will see the truth.

And what I also pointed out is that O&O is a vestige of the non-locking oval carabiner days in which this was the only way to create a secure top-rope anchor. A single locking carabiner, properly used, is as safe as two O&O non-lockers, which is why we use a single locker on our harness for belay and rappel, at the belay station and everywhere else that security is required.

No, the primary reason we use 2 biners is for strength and redundancy.

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Edit: I missed something with the last comment above. I was assuming gates on opposite sides but both biners oriented normally. I can't see any good reason for also "opposing" them, if that is the appropriate term, and it is true that in that case the narrow end of one of the "D's" will be part of the rope-bearing surface, potentially contributing ever so slightly to additional sheath wear.
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I use two because I bought two matching carabiners long ago, and if I only used one I would have to carry around the other and think up an explanation for why I own it.

Also, I am skeptical about the "screw down or you screw up" theory of gravity. I would imagine the weight of the locking ring to be negligible in comparison to the forces that might turn it in either direction.