Traditionally mechanical slide carbs have always had 2 cables to close the throttle. with mechanical slides the engine vacuum can in some cases pull the slide tight against the body of the carb preventing it from closing effectively, by using a 2nd cable to close it overcomes this problem.
CV carbs can usually get away with a single cable as the butterfly will close with a lightweight spring, and the slide doesn't stick like a mechanical slide will do.
Expensive carbs like the Mikuni TMR range or the Keihin FCR range have the slides run in roller bearings which overcomes the problem of the slide being wedged against the carb body by engine vacuum. My Mikuni TMR flatslide have 4 small roller bearings that the slide operates on, I have run these carbs with a single throttle cable for the last 6 years with no problems so far. My Mikuni RS36 flatslides run with both an opening and closing cable as they are prone to sticking under certain conditions.
If fitting your second cable is causing problems then it sounds like the cable inner liner is worn causing the inner to bind internally, rather than a cable routing issue. Buy yourself a new cable, and get yourself a cable lubricating tool, and use it on a regular basis when you perform a service on your bike, it will make for a much smoother throttle action, and you'll be surprised how much lighter a clutch action can be with a regularly lubricated cable. They are not expensive, and are easy to use, remove the top end of the cables and fit the cable oiler, then squirt a can of WD40 or similar into it until the fluid coming out of the other end runs clear, meaning it has flushed any crap out of there. A simple 5 minute procedure which is usually overlooked until a cable snaps or seizes.
