I would suggest this to be an absolute NO-NO! The cases are machined as a matched pair meaning, exactly as Steff750 said, that the main bearing housings WILL be mismatched / mis -aligned.
Out of interest though, the only difference between 9 hole and ten hole clutch covers is the extra hole. A ten hole clutch cover WILL fasten directly on to a sandcast crankcase set, except of course you have an extra hole that you can't put ascrew through. With careful modification and a tiny bit of welding, a 10 hole cover can be coverted to look like a 9 hole cover, if necessary. Its all good fun!
Chris R.