In reply to Andy Morley:
This is what I said then
Abbed off 6mm+9mm, 7mm+8.5mm, 5mm+5mm .
Be careful with differential rope slippage, slashing thin ropes and rope stretch.
Ideally practice with a top rope first.
Getting the ideal amount of friction takes practice.
If possible I put both ropes thro the same hole in the plate and if the rope is very thin an additional friction hitch after or before the plate.
On a 20m free ab on the double 5mm, an in line Italian hitch and ATC worked very well, but otherwise felt on the limit of acceptability for safety, not for the faint hearted. I still prefer double 10mm.
Post edited at 17:41