Doug, it is the flexibility of, say, a nylon fabric that spreads or distributes force throughout a lap or flat felled seam, or even a faux felled seam. Perhaps not so much with less flexible, non woven dyneema fabrics, but think the force on one line of stitching would still be shared with the other line.
One has to be creative with pack seams. Even with tunnel tents, Roger Caffin has posted illustrations of the seams he designs for attaching pole sleeves. Pack fabric is heavier, so one can take liberties in designing the seams; but I think two rows of stitching are better, if only for the reason you mentioned; to hold things together if one seam fails.
It is probably just as important to have some seams that show only internally, as the exterior of the pack is where the abrasion occurs that can shred a seam. I seem to remember there are articles, or at least links to articles, on BPL about seams. I get the impression from the forums that many are joining dyneema fabrics with tapes these days. I think it is a matter of ‘do what you know’ to get the best results.
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