As a service manager, I am always conscious of costs on customer's vehicles. If you are doing the job yourself, I would just replace the torn boot as long as the CV joint is still good; I would only replace the shaft if the joint was pooched. I would not incur the added expense of the second shaft if the boots on it passed inspection. Boots are generally only torn or not. They are not really a preventative maintenance item and rarely a wear item. Save your money - $15 boot kit vs $125 shaft.