I am myself investigating the re-enabling the tach on my car. When the time comes.
I do not think you need a double shaft.
It depends on the equipment you have but as far as I know...
The electronic tach are an electronic counter. The tach count the pulse from the ignition coil. 4 cylinders two pulse per rotation. 1000 rpm equal 2000 pulse.
The only reference material I have is for the Pontiac Fiero, I beleive my tach on the Pontaic Sunfire will be a modernized version. I will know when I find the references or when I open the thing up.
Aparently some systems will have a field (magnet) and will count the teeth on a gear, like the flywheel's. That could be one pulse, for one rotation.
I have seen an EV, somewhere, the owner had an homemade optical system with white and black marks on the tailshaft. The same would work inside the bell housing using the starter opening. I do not have the technical knowledge for such a device. I can follow instruction, I cannot conceive such things.
I myself will investigate using rare hearth magnets and reed switches. I am not shure how fast they close. Yet.
Two magnets, two pulse. Just like my 4 cylinders car. Plus that would keep the shaft balanced.