Take one 10-15' fall on a clove hitch and you'll seize the knot up tighter than a 2000 yr old witches nipple. You'll then have to cut the rope... trust me. Been there and done and it SUCKS!!!
Get one of these...
Or learn how to adapt and use one of these....
I use both on a regular basis...
Why would you want to use TWIN ROPES?
You want to trail a 9-11mm static zip/rap line equal in length to your primary in order to jug/haul on anyway.
Counter rapping/hauling is blast and far more efficient any day....