I just switched the freehub of my Ksyrium rear wheel from a Shimano (M10) one to a Campagnolo (ED10). The M10 had two spacers that went on the freehub in between the spokes and the Shimano 10-speed cassette. Only one of those fits over the Camply splines. Do I need to use this spacer when I put on the Campy cassette (Centaur 12-25)?
So you are talking about a thick spacer and a really thin 1mm spacer?
The 1mm spacer is only for shimano 10speed cassettes. The thicker mavic spacer is for shimano cassettes. I dont think you need that spacer. I'm pretty sure you will not be able to put on the lockring if you use that spacer.
Campy cassette bodies are wider than shimano. The spacer makes up for this width.
On your old setup the thicker spacer was to make up the difference between a Shimano cassette and a MAVIC cassette, the thinner spacer was for the difference between a Shimano 10-speed cassette and a Shimano 8 or 9-speed cassette, the 10-speed cassette is narrower. Not sure what you need to mate up a Campy cassette with a MAVIC Campy compatible freehub. My guess is no spacer, I don't think MAVIC cassettes were built with the Campy spline pattern.