I've begun actively looking at twin ropes. The lightest I found was the 7.5mm Twighlight by Mammut. But, this is below the recommended limit of my ascenders (8mm minimum). When I posed the question to Petzl, this is what they said:
Hello Greg,
There is no question that the handled ascenders will grab a Twilight rope.
However, Petzl has not done any testing on a 7.5mm rope. As a company line
we can only recommend a rope down to 8mm. If you use the rope grab on rope
thinner than 8mm you are doing so at your own risk and without Petzl's
recommendation. I would suggest that you take your rope to a good
specialty shop with the capability to allow testing of your setup before
purchase. I hope this helps Greg. And as always stay safe and know your
system.
Crandall Caughman
Petzl America
800.686.6206
ccaughman@petzl.com
Going to my local shop, I used a 7mm Beal rope to witness that a Petzl ascender would grab a rope that small (unweighted), but could not test a weighted rope. Even with an 8.1mm twin rope, this seems pretty sketchy with all the rope stretch and the probable amount of quickly induced ascender related wear on the rope.
So, is there a good way (prussiks) to ascend twin ropes in the event that the pitch is too difficult for the second to free climb?
If in doubt about the second being able to free climb the pitch, would half ropes (juggable) be recommended?
Greg