Best way to sync collinear Spindles (running as step/gen signal)?

This is the second part to my plan of converting my taig ball screw lathe to a Swiss-style lathe. I need to sync both the main drive servo (Which is on the new Z axis BEHIND the taig headstock, and a second servo that rotates the guide bushing inside the headstock. (the native taig Z axis will be used for W offsets of gang tools). I know the best way to do so is mechanically but I think it would be too hard to get up and running and am hoping to get some results sooner. Anyway....


Do you all have ideas on the best way to do this? Right now I run any spindles on the lathe as step/gen instead of analog. Currently on my regular taig, my rotary encoder is on the 5C headstock shaft

My plan: Move the rotary encoder onto the drive servo so that it correctly shows the rotation of the bar stock itself. Then...
(1) Potentially try to split the rotary encoder signals - one to the linuxcnc mesa board, and the second directly into the encoder input on the slave servo driver
(2) Split the step/dir signals that normally go into the master but split them to go into the slave servo and just ensure my drive settings like PID loop is all identical (encoder stays on the main drive shaft of the master spindle and its signal goes solely to mesa)

Some notes:
- Tandem axis would not work for this I believe
- I am using a low resistance guide bushing, meaning it is easy to spin the stock through the guide bushing, so I suspect as long as the RPM's are relatively similar it should not cause a big issue like stock twisting or serious marring
- The motors I am using would be a master 1000W A2M-80SV03230 servo motor with S2SVD20 drive and a slave 400w servo motor of same type with driver S2SVD15 which will simply drive the headstock to rotate at the same speed as the stock (both are from RATTMOTOR)

Easiest way is to connect the main spindle PID-out signal to both stepgens (main and guide bush drives).  You can do the same for the drive enable signal.

Make sure to program both drives with the same input steps/rev.

If you want to be able to un-couple the guide bush motor for some reason, you can have an AND2 component between the PID signal and the bush stepgen which requires some other thing to be true (like a switch, button, or M-code enabled pin).

If you want a fault alarm in LCNC for the guide bush drive, you can add a misc-error and connect the bush drive alarm output.  See the MOTION component man page.
linuxcnc.org/docs/devel/html/man/man9/motion.9.html