Thanks for the reply - I did look into whether there were any off the shelf CNC clutch mechanisms that I could buy but couldnt find anything. I mean I could probably design something, or use e.g. lawn mower clutches but it is still essentially using steppers to mechanically engage each drive, one at a time. Which I can do, I just thought maybe there was another way! The only alternative that I saw elsewhere, was a suggestion to use a variable reluctance stepper, which is happy to freewheel, but my inclination would still be to disengage the main motor when the stepper is active to reduce mass being driven and backlash. At which point I might as well put both drives on a clutch. Perhaps I am being over the top with it though, and over-thinking the problem? Anyway, the interlock is an interesting idea, I hadnt thought of that!