Both sentences mean the same thing. They are combined to emphasize that Jesus taught the crowds only with parables. (See: [[rc://en/ta/man/jit/figs-parallelism]])
"he taught them nothing except by parables." The double negative can be expressed in a positive way. Alternate translation: "everything he taught them he said in parables" (See: [[rc://en/ta/man/jit/figs-doublenegatives]])