Here is my idea for dual sprockets. Please feel free to inform me what won't work!
I would like to run dual front sprockets only. 13 and 15 tooth with my stock 50 tooth rear. I would also like to be able to change from one to the other only loosening the rear axle, not taking the chain apart. My thinking is that if its too much work, I just won't change sprockets very often and won't be taking advantage of the setup.
Is there enough room in the side cover for the chain to go over the top of the 15t sprocket and slide over to the 13t?
If you are going to spend the time to do a dual front, you might as well do the rear also for even more flexibility. I did both front and rear and with this, chain alignment is perfect for each 'set'. The 13 will be tough to index as the diameter is to small for screw hole for securing the mated pair. The primary shaft splines are only long enough for one sprocket, the sister sprocket will need the splines removed so it can slide over the shaft. My first set I did in my shop on a drill press with grinding stones (cone). I suggest you make the splined position on the shaft available for your off-road/trail gear combination. I was able to do six teeth without having to worry about adding chain sections to accommodate my sprocket combinations (15-50 & 14-55). Good luck. I am as well working on another front combo.