If using the Demon Post processor as is, you could use either. But disconnect both from the Y steppers or one side will try to move. That post processor uses Y as the rotational axis.
I have contacted Vectric and here is their reply: Your machine manufacturer / supplier will be able to supply you with an example output file which is in the form of a calibration file used to verify that the machine is setup and working as expected. Once you have been able to successfully test your rotary setup, if you could then please send this over to us then we should be able to take a look at the possibility of providing a post which creates equivalent output for our software. In absence of this, we have no knowledge of what your machine requires or any verification that the machine is setup and working correctly with it's own control software so post creation at this point would not be possible I'm afraid. How do I proceed?
Thats quite unhelpful. Not sure how to proceed either - though I would want to suggest avoiding Vectric then and thinking about Fusion (Post already here) or DeskProto offers CNC machining for non-machinists They have a EUR248 version DeskProto Multi-Axis Edition, Hobby license
Before I commit to DeskProto I would like to try writing a PP for my machine. Please suggest references on writing the PP.