Do not know about C permit, but for B permit there is an exception. If you are married to someone with a B permit it is possible to have two permits.
I know somebody that worked at CERN with a "permis fonctionaire" and was married to a person with a B permit.
They wanted to buy a house but he was not allowed to with his permis fonctionaire.
If he would quit CERN he would get a B permit and would be a able to buy a house.
With the help of the Swiss consulate in Geneva he eventually got two permits, but the process was cumbersome. He received several letters that he could pick up his permit only to find out at the office de population that he could not get two permits.
So yes you can have two permits if having just one leads to strange situations (like quitting your job to find a place to live so you can actually do your job

)