If you're trying to separate the two halves of the coupler, what about using a cold chisel and mallet right along the two mated surfaces? If you're going to replace the coupler anyway, I don't think it would be too big a deal if it was slightly gouged. Also, since the only thing holding the two halves together is rust, have you tried tapping straight down on the shaft side of the coupler? Shouldn't need too much force to break that apart. Tap, rotate, tap, rotate until free.