CNC lathe question

I am trying to build a CNC controlled router to mount onto a
woodturning lathe. It will hopefully do shaping but I am also hoping
for thread operations for barley twists etc. Does it seem reasonable
to have 2 motors, one variable speed spindle motor for normal lathe
operations and another stepper motor to do the threading operations?
Would it be possible to do threading with the variable speed
controller
and spindle motor alone. It seems this would require a very accurate
knowledge of the RPMs to time the travel of the carriage. I am
thinking that a stepper motor would be accurate enough, but probably
not turn fast enough for normal lathe operations.

I know how it works on my manual metalworking lathe, but I will not
have the gearing that lathe uses.

Thanks for your input,

Keith Clark