We took our first 5th into Mexico a few years ago - registered to husband - didn't return the permit - and then sold it in Canada. We now have a different 5th, and are returning to Mexico in January ...
Our motorhome is registered in both names (B.C.) but the permit is only in one name and there was no issue about the second name on the registration. That being said the motorhome permit is in my name and every year I get a permit for the tow also in my name so in fact I have 2 permits - 1 10 year and one 180 day and it has never been a problem.