Ok... Mach will only use one motion device. You cannot spread coordinated motion axes across two or more plugins. For spindles, you can use any modbus device (Galil RIO) or any plugin that controls an analog output (Maybe PoKeys does this?). The deal is that a spindle is not a coordinated motion axis in most instances. Threading and rigid tapping excluded, of course. So if you just want RPM control of your spindle, you have other options.
For a feed type axis, you could also implement a motor with a VFD speed control. Instead of using a Mach axis, you could do a special script M code that moves the feed motor at a certain speed.
Steve