Carrying two tubes is fine but you really only need one. The 19" front tube can be stuffed into the rear tire if needed, on dirt bikes we only carry a front (21") tube that fits fine in the 18" rear tires. I guess I also learned that the smaller tube can be stretched out over a larger rim!
I've never had to use a tube on the Tenere, but you'll also need some way to remove the valve stem if you're in that situation. Sharp knife, side cutters? I didn't really think about it, but I guess it could be another issue if you have to use a tube.