Did I hear you correctly? I am imagining you tying two pieces of rope together and then rappin on a double strand. You say 10mm & 8mm. Where do you draw the line on dia. differences and why not just block it and rap single line or use a contingency anchor? If I'm using pull cords my set up would be 9.3mm and 6mm (obviously static) and I can't imagine wanting to rap that set-up as a double line. I rap on different diameter ropes all the time. Very rarely do I see them move. Free online calendar with sync to Outlook(TM).