May be an Idea to start a Company, making what you want, and perhaps others may too..

Ideas:
a tandem axle is available .. 145 * is a common length, 10mm x 1 thread per MM,,is Shimano's , and thus a common thread spec.

then you have to sleeve it out, to 12mm for bearing support so a threaded sleeve needs making...

Its solvable.. with a machine shop..

*There are longer as well..




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