Due to fear of spoke breakages on long rides, I bought a
FiberFix kit from Peter White Cycles last year. It comes in a tiny cylinder and weighs just a few grams. I found that you can also squeeze a quick-link for a chain into the same pot to keep things tidy in your saddlebag. Fortunately, I haven't had the opportunity to try it out yet, although I did read through the instructions carefully to make sure I understood it before taking it on the road. You can install it in any position without removing the cassette. I don't think it's officially "tandem rated," but if you've broken a spoke then using it is going to be much better than using nothing.
Proper touring cyclists will carry a set of spare spokes, often taped to their left chain-stay. You can then carry a small tool called the Next Best Thing 2 (
NBT2) to remove the cassette when needed. See
this page for other similar options.