Cheesewhiz said:
Just because you can do something, doesn't mean it's a great idea.
If that off caliber magazine would jam up, it could damage that magazine and then you would have a magazine that won't work for its intended rounds any longer.
The other consideration is if you had a magazine holding the wrong rounds and mixed up and loaded it into the the other pistol, you would have a problem there. Mixing things at the range is never a good idea, I've seen it done and the results are not good.
I always find peoples mistakes while cruising for brass. I usually pick them up and take them home and I have a little bucket for them. I usually spend a little time trying to run forensics on them to determine exactly what happened. I couldn't find my bucket off hand but I had this one laying on the bench handy for pics.....223 headstamp fire formed in a yet to be determined chamber. Shows a badly offset primer strike [just barely ignited] split neck, shoulder, and catastrophic failure in the body. This made a mess of someones day if I had to guess.
Back to the OP,
We had this 9mm vs 40 cal P series magazine discussion at length several years back. There was a very pleasant and knowledgeable member here that went by COFFEEPOT. Many posts here have disappeared over the years but you may be able to search his old posts regarding the subject.
IIRC, there were body, feedlip, follower differences and the mags went through several design changes. The original no crimp bodies allowed loaded mags to rattle which annoyed some. The 1994 AWB reduced the capacity 1 round to comply which brought about the Crimped bodies. If I remember right switching the mags was not recommended as a general rule because all of the variances throughout the years in both the P17 40 cal and P15&P18 9mm mags.
So you may have an early P17/11 that works perfectly and you trust but when you buy a different P17-10 it may not be very reliable. Or worse yet, it may be 99% reliable which only allows for a Murphy moment.
I would recommend just getting the appropriate mag to remove the variable and just as cheap insurance. I do share your fondness of interchangeability but in this case I always just bought separate mags for the 40 and 9mm PC carbines and P series pistols. Good luck and let us know how they work with live fire loaded to full capacity.