I've leash trained cats, but as Pattymac said it's mostly about following the cat around to where cat wants to go. At the time we had a 1/2 acre property and cat was content to walk the perimeter of the property, and yes some gentle pulling in a different direction worked about half the time. Off our property the cats were very insecure and a dog or motorbike going by would freak them out. I don't want to put a damper on your dreams of walking dog and cat together, but I think it would be very difficult. A loose strange dog comes up, a bunch of kids running down the sidewalk, or motorbike and cat freaks out? Then what? That kind of situation would be very difficult to manage, as some cats just go berserk in trying to get away and may bite, or worse wriggle out of their harness and run off!
What you could do is buy a Pet Stroller for the cat, and then you could take your dog for a walk at the same time. Most pet stores sell them.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous