In reply to Greasy Prusiks:
> The thing I can't get my head around why having such a large angle between the rope and the gear doesn't cause the force to increase.
Three reasons:
1) The ropes won't be super tight in the first place so the 'high angle' is slightly theoretical.
2) In addition to any slack, the
full pitch length of dynamic rope will stretch significantly in any fall, further reducing the angle. [This is the main difference compared with a belay and you may have only been thinking about the couple of metres of rope between the closest runners and forgetting about the other 30metres or so of rope in play.]
3) Any standard belay device is intrinsically load limiting and will slip at 2-3kN anyway.
As such, any forces will certainly be no higher than those experienced in any moderate vertical lead fall.
However, although the second won't take a big swinging fall, if they do come off they're quite likely still to drop a good 2-3metres or even 4metres depending on the route and gear spacing albeit with a very soft catch.