I have a puller too. And you're right - if you put them on a coupler, they drag the other one around.
I've started taking her on a separate walk in the evening. It takes us FOREVER because everytime she pulls, I stop & make her wait until there's no more slack on the leash. But she's really getting better. It's just extremely tedious. I also got her a Bungee Puppee leash. That didn't work out so well. It's supposed to discourage pulling, but it does the opposite for us. Maybe it works better with larger dogs.