Martin,
One more possibility...
There is a place in New Jersey, State Electronics, that apparently is making modular potentiometer assemblies similar to the ones used in Tek equipment.
Here is a URL to their "Mod Pot" catalog:
The catalog looks like they can build to order from stock pieces, they do list multi-section potentiometers with dual concentric (.125/.078 inch) shafts.
I don't know how spendy, however it could be an option. The catalog talks about supplying prototype quantities.
gwerl
