No idea what drives you're using but most servo drives have an encoder output. So you connect the physical encoder to the drive, then the drive emulates the encoder output and you connect that to the motion controller. As mentioned it's not wise to try to split an encoder signal but Mesa used to sell an "encoder Y" board that splits the encoder into 2 outputs if you really wanna do it.
That said I don't really see the point. The drive itself is closing the feedback loop so the controller doesn't have to. It should be sufficient just to command the drive and just wait for the output signal that says "I didn't do what you told me to"
You could get some benefit from using they closed loop stepper as it's meant, but also adding another encoder to the axis and feeding that back to LinuxCNC. For example, if your doing a Mill you could couple an encoder to the end of the screw on the opposite end of the table. That way if the motor coupler slips, the closed loop step drive won't catch it since the motor did spin, but the other encoder on LinuxCNC would see it.
