I recently changed from a triple chainset to a double, because the axle on our triple is longer than that of a double I bought two secondhand double chainsets from eBay and cut and welded the two axles to make them the same length as a tripple. It also meant that I went from a 48 38 28 130 BCD to a 48 33 110bcd. Our tandem came with 5mm of spacers on either side of the bottom bracket, and needs them because the Gates belt drive would rub the frame. I guess you could go narrower if you either converted to chaindrive and or spaced the crossover chainrings out further.
All this so we could run hydraulic brakes with standard brifters.