novocaine
Retro Wizard
We need to increase the pull from 2x20 (assumed - the double lever is advertised as being for cantilever, and the OP is applying it that way) to a single 40mm v- brake pull to satisfy @Imlach 's "fixing cantilevers for ever"
Op already has double cable pull lever, and the bmx splitter used as a coupler would pull the same length in and out.
I think we might need a 3d printed leverage device?
2 in to 1 then a travel agent?
