Bear with me, because I know the answer to this is technically "don't". However, I have a Sturmey Archer equipped tandem, and I just cannot find a tandem length shifter cable. It arrived with 2 single cables joined by a piece of electrical terminal block, but that's failed twice on me now, so I need a better solution.

Now, I'm trying hard to find a real cable, but in the absence of that, how could I join these in a way that wouldn't slip or seperate?