I've thought long and hard on this one. I've not thought about any of the exotic fibres as it's easy enough to make a durable jackline that still much exceeds the strength of your tether and harness.
Possibly a single long would, in the unlikely event that it broke as you tumble overboard, give enough slack to hang behind the boat and be the Hail Mary line that might save you. On the other hand, two seperate lines are considerably easier to coil up and stow when not-off shore. I have gone with the latter but I don't know that it's a profound difference.
I feel more strongly on not making the jack lines tight. When you fall you're loading the lines about normal to their run. The smaller deflection you'd have with the jack lines not having any slack means far greater strain on the anchors and on the lines. I'd rather have a little more travel in my fall and keep the strain on the system down a little.
I run my jacklines as near to center as practical, as avoids hanging up on vents and such, for a couple of reasons. One is that up along the cabin house puts it less likely to be under foot.
Another is that the more centered run makes it relativly easier to arrange the tether length such that you can get to most everything - even a lot of stuff on the other side of the boat - and still have a length that will keep you about at the rail rather than dragging along in the water. Having experimented with attempting and failing to get back in a boat even moving at a sedate 4 knots when held amidships and fully in the water, I am convinced this matters.
Finally, I have used both round line and flat tape for jack lines. The latter is less of a problem if it does get under foot. Round jacklines have an advantage in that more people already know how to make a perfectly good splice, while the standards for stitching a loop in the end of a tape might take a little looking up.
G'luck
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