The rear cable between both cables is a single cable. If you have disconnected the cable on one side, it will be loose/will not work on the other side. If one caliper is sticking on with the cable connected, it’s because something is not releasing. You probably need to fit a new rear handbrake cable and you may also need a caliper but try the cable first. Can you operate the sticky caliper lever with your hand and does it move freely back? If not, it’s the caliper. Until you have done this and can show a picture of the new cable fitted, I won’t be giving you any further advice on this car that aids you driving it on the road. Sorry, but Andy is right - if you can’t fix it, get it off the road. You are breaking the law and endangering others by knowingly driving a vehicle on the road with faulty brakes.