I WOULD USE AN AUTOBLOCK(PRUSSIK, KLEIMHIESHT, ETC...) INSTEAD.
BEEST THING TO DO IS NOT PERFORM DOUBLE ROPE RAPS WITH SUCH A SETUP.
TIE THEM TOGETHER. PLACE THE KNOT AGAINST THE CHAIN, RAP RING ETC.... AND RAP THE FAT ROPE. HAVING THE SMALL ROPE FOR PULLING ONLY. ALSO CLOVE HITCH A BINER ON THE FAT ROPE BELOW THE RAP RING AND CLIP THE SMALLER DIA. ROPE THROUGH(THIS IS ESSENTIALLY A BACK UP)
EMAIL ME FOR MORE CLEAR DIRECTIONS...OR I CAN SHOW YOU NEXT WEEK.