Dual tachometer cable

We are building a two place biplane with mechanical tachometers in both cockpits. The engine is a Lycoming O-360. We have a dual cable splitter that we will run on the front tach and have the cable that goes from the engine to the splitter. What we need is a source for the cable running from the splitter to the second tach. This is a square drive to a square drive. All we are able to find is the engine style cable that is a key drive to square drive. Any help in sourcing this would be appreciated. If we are doing this wrong, then what are people using to drive two mechanical tachs?

Thanks,