I am planning on making the nipples from brass, a good friend has a nice little hobby lathe I can use. I will open one end out like this (V) so the New stainless inners can be splayed before soft soldering in place. I hate to say it but Shimano Durace outers are a good match for the later silver/grey Resilion outers.
What is the Tandem you are working on? I have one in the Garage awaiting a renovation. Milne from the 30's Has resilion front and rear, should also have a rear drum (no wheels with it) and Cyclo 3 speed. Which will also be fun to make cables up for.
You may also like to try Venhill engineering a friend uses them for Classic Motorcycle cables, that they make to his spec.
Steve