eric, I hope we're not hijacking your thread, just seems to be growing.
digits, I believe your concept can be done. Might be a bit long though. You could have a hollow shaft gear box that would give you lathe speed and a second gearbox to provide indexing. A central shaft running through both, as you mentioned. The central shaft (aka spindle) suspended by its own bearings, and engaged by hydraulic clamping sleeves (Spieth makes them). Would have to be safely set up so both could not be engaged at the same time.
Yes, very Rube Goldberg and impractical. Also, probably expensive as well. Might even qualify for something for a Kostner sequel to "Water World". lol