In reply to mrjonathanr:
Personal preference, maybe, but there is a final answer in my book, but apply mainly to sport routes and a lot less to trad routes...
If say, you clip a bolt and traverse left from it, the gate of the top karabiner should face right - to prevent the bolt snagging in that little notch next to the gate. (See the thread about testing little krabs recently - yes, I know that was when used with wires). The lower krab should also face right to prevent it from coming into contact with the rock (the lower krab, to my mind, is slightly less important than the top one.) If the gates face different ways, you can't achieve that.
And yes I've held a fall where a krab snapped in a bolt when clipped the 'wrong' way. It was a big chunky Simond krab, not a tiny lightweigh job.
Just three observations: 1. On overhanging routes the orientation of the lower krab is of little importance. 2. The snagging I referred to is not likely to happen when clipping big glue in rings. 3. I think when trad climbing there's every reason to have both krabs free and the orientation doesn't matter so much.