I went through the same problem when I needed a new (second hand) n/s shaft last year. Held side by side, left and right, they look the same even though the Haynes manual says they are unequal length. Good advice re the wind up stress, I would always stick to keeping them to the same side however if all you can get is what is available then you go with that. After all, a pair of reconditioned shafts, unless they are marked left and right will most likely be wrapped together and you just put whichever on whatever side. I suppose getting them 'mixed up' would only show up under the most arduous of conditions as regards the winding/ unwinding. Think of them like a piece of thick rope - grab each end and twist then turn it backwards.