I thought he was thinking of of using a servo only for both -
Yes and that would be the problem, the CSMIO controller would command the spindle but as the encoder is shared with the C (same motor, same encoder) then it would try and stop the C axis moving so it would put out an analogue voltage out the same channel as the spindle, that would not be a healthy situation. It would be similar to you commanding a spindle speed and an axis speed at the same time and worse still they would be opposites. Even if that problem was got around by using different channels and relays and you did likewise with the encoder signals then as soon as you swapped the channels via relays the encoder signals would vanish and either the spindle or C axis would fault due to loss of feedback, in fact the whole control would E-Stop I would think.
if he did go with a magnetic clutch wouldn't all power to the spindle have to be off during indexing -
bill
Yes that would likely have to happen so probably two clutches would be needed. It may be possible to just disable the VFD, if the Indexing is relatively slow then I don't think there would be any regen issues but I am no expert on such things so the two clutches would be my preferred option.